[DOC] Use RDoc inclusions in string.c (#5683)

As @peterzhu2118 and @duerst have pointed out, putting string method's RDoc into doc/ (which allows non-ASCII in examples) makes the "click to toggle source" feature not work for that method.

This PR moves the primary method doc back into string.c, then includes RDoc from doc/string/*.rdoc, and also removes doc/string.rdoc.

The affected methods are:

    ::new
    #bytes
    #each_byte
    #each_line
    #split

The call-seq is in string.c because it works there; it did not work when the call-seq is in doc/string/*.rdoc.

This PR also updates the relevant guidance in doc/documentation_guide.rdoc.

5 files changed, 204 insertions, 0 deletions

diff --git a/doc/string/bytes.rdoc b/doc/string/bytes.rdoc
new file mode 100644
index 0000000000..a9e89f1cd1
--- /dev/null
+++ b/doc/string/bytes.rdoc
@@ -0,0 +1,6 @@
+Returns an array of the bytes in +self+:
+
+  'hello'.bytes # => [104, 101, 108, 108, 111]
+  'тест'.bytes  # => [209, 130, 208, 181, 209, 129, 209, 130]
+  'こんにちは'.bytes
+  # => [227, 129, 147, 227, 130, 147, 227, 129, 171, 227, 129, 161, 227, 129, 175]
diff --git a/doc/string/each_byte.rdoc b/doc/string/each_byte.rdoc
new file mode 100644
index 0000000000..643118fea3
--- /dev/null
+++ b/doc/string/each_byte.rdoc
@@ -0,0 +1,17 @@
+Calls the given block with each successive byte from +self+;
+returns +self+:
+
+  'hello'.each_byte {|byte| print byte, ' ' }
+  print "\n"
+  'тест'.each_byte {|byte| print byte, ' ' }
+  print "\n"
+  'こんにちは'.each_byte {|byte| print byte, ' ' }
+  print "\n"
+
+Output:
+
+  104 101 108 108 111
+  209 130 208 181 209 129 209 130
+  227 129 147 227 130 147 227 129 171 227 129 161 227 129 175
+
+Returns an enumerator if no block is given.
diff --git a/doc/string/each_line.rdoc b/doc/string/each_line.rdoc
new file mode 100644
index 0000000000..e254c22d40
--- /dev/null
+++ b/doc/string/each_line.rdoc
@@ -0,0 +1,60 @@
+With a block given, forms the substrings ("lines")
+that are the result of splitting +self+
+at each occurrence of the given line separator +line_sep+;
+passes each line to the block;
+returns +self+:
+
+  s = <<~EOT
+  This is the first line.
+  This is line two.
+
+  This is line four.
+  This is line five.
+  EOT
+
+  s.each_line {|line| p line }
+
+Output:
+
+  "This is the first line.\n"
+  "This is line two.\n"
+  "\n"
+  "This is line four.\n"
+  "This is line five.\n"
+
+With a different +line_sep+:
+
+  s.each_line(' is ') {|line| p line }
+
+Output:
+
+  "This is "
+  "the first line.\nThis is "
+  "line two.\n\nThis is "
+  "line four.\nThis is "
+  "line five.\n"
+
+With +chomp+ as +true+, removes the trailing +line_sep+ from each line:
+
+  s.each_line(chomp: true) {|line| p line }
+
+Output:
+
+  "This is the first line."
+  "This is line two."
+  ""
+  "This is line four."
+  "This is line five."
+
+With an empty string as +line_sep+,
+forms and passes "paragraphs" by splitting at each occurrence
+of two or more newlines:
+
+  s.each_line('') {|line| p line }
+
+Output:
+
+  "This is the first line.\nThis is line two.\n\n"
+  "This is line four.\nThis is line five.\n"
+
+With no block given, returns an enumerator.
diff --git a/doc/string/new.rdoc b/doc/string/new.rdoc
new file mode 100644
index 0000000000..3eee2b82e0
--- /dev/null
+++ b/doc/string/new.rdoc
@@ -0,0 +1,37 @@
+Returns a new \String that is a copy of +string+.
+
+With no arguments, returns the empty string with the Encoding <tt>ASCII-8BIT</tt>:
+  s = String.new
+  s # => ""
+  s.encoding # => #<Encoding:ASCII-8BIT>
+
+With the single \String argument +string+, returns a copy of +string+
+with the same encoding as +string+:
+  s = String.new('Que veut dire ça?')
+  s # => "Que veut dire ça?"
+  s.encoding # => #<Encoding:UTF-8>
+
+Literal strings like <tt>""</tt> or here-documents always use
+Encoding@Script+encoding, unlike String.new.
+
+With keyword +encoding+, returns a copy of +str+
+with the specified encoding:
+  s = String.new(encoding: 'ASCII')
+  s.encoding # => #<Encoding:US-ASCII>
+  s = String.new('foo', encoding: 'ASCII')
+  s.encoding # => #<Encoding:US-ASCII>
+
+Note that these are equivalent:
+  s0 = String.new('foo', encoding: 'ASCII')
+  s1 = 'foo'.force_encoding('ASCII')
+  s0.encoding == s1.encoding # => true
+
+With keyword +capacity+, returns a copy of +str+;
+the given +capacity+ may set the size of the internal buffer,
+which may affect performance:
+  String.new(capacity: 1) # => ""
+  String.new(capacity: 4096) # => ""
+
+The +string+, +encoding+, and +capacity+ arguments may all be used together:
+
+  String.new('hello', encoding: 'UTF-8', capacity: 25)
diff --git a/doc/string/split.rdoc b/doc/string/split.rdoc
new file mode 100644
index 0000000000..d93b76d9b4
--- /dev/null
+++ b/doc/string/split.rdoc
@@ -0,0 +1,84 @@
+Returns an array of substrings of +self+
+that are the result of splitting +self+
+at each occurrence of the given field separator +field_sep+.
+
+When +field_sep+ is <tt>$;</tt>:
+
+- If <tt>$;</tt> is +nil+ (its default value),
+  the split occurs just as if +field_sep+ were given as a space character
+  (see below).
+
+- If <tt>$;</tt> is a string,
+  the split ocurs just as if +field_sep+ were given as that string
+  (see below).
+
+When +field_sep+ is <tt>' '</tt> and +limit+ is +nil+,
+the split occurs at each sequence of whitespace:
+
+  'abc def ghi'.split(' ')         => ["abc", "def", "ghi"]
+  "abc \n\tdef\t\n  ghi".split(' ') # => ["abc", "def", "ghi"]
+  'abc  def   ghi'.split(' ')      => ["abc", "def", "ghi"]
+  ''.split(' ')                    => []
+
+When +field_sep+ is a string different from <tt>' '</tt>
+and +limit+ is +nil+,
+the split occurs at each occurrence of +field_sep+;
+trailing empty substrings are not returned:
+
+  'abracadabra'.split('ab')  => ["", "racad", "ra"]
+  'aaabcdaaa'.split('a')     => ["", "", "", "bcd"]
+  ''.split('a')              => []
+  '3.14159'.split('1')       => ["3.", "4", "59"]
+  '!@#$%^$&*($)_+'.split('$') # => ["!@#", "%^", "&*(", ")_+"]
+  'тест'.split('т')          => ["", "ес"]
+  'こんにちは'.split('に')     => ["こん", "ちは"]
+
+When +field_sep+ is a Regexp and +limit+ is +nil+,
+the split occurs at each occurrence of a match;
+trailing empty substrings are not returned:
+
+  'abracadabra'.split(/ab/) # => ["", "racad", "ra"]
+  'aaabcdaaa'.split(/a/)   => ["", "", "", "bcd"]
+  'aaabcdaaa'.split(//)    => ["a", "a", "a", "b", "c", "d", "a", "a", "a"]
+  '1 + 1 == 2'.split(/\W+/) # => ["1", "1", "2"]
+
+If the \Regexp contains groups, their matches are also included
+in the returned array:
+
+  '1:2:3'.split(/(:)()()/, 2) # => ["1", ":", "", "", "2:3"]
+
+As seen above, if +limit+ is +nil+,
+trailing empty substrings are not returned;
+the same is true if +limit+ is zero:
+
+  'aaabcdaaa'.split('a')   => ["", "", "", "bcd"]
+  'aaabcdaaa'.split('a', 0) # => ["", "", "", "bcd"]
+
+If +limit+ is positive integer +n+, no more than <tt>n - 1-</tt>
+splits occur, so that at most +n+ substrings are returned,
+and trailing empty substrings are included:
+
+  'aaabcdaaa'.split('a', 1) # => ["aaabcdaaa"]
+  'aaabcdaaa'.split('a', 2) # => ["", "aabcdaaa"]
+  'aaabcdaaa'.split('a', 5) # => ["", "", "", "bcd", "aa"]
+  'aaabcdaaa'.split('a', 7) # => ["", "", "", "bcd", "", "", ""]
+  'aaabcdaaa'.split('a', 8) # => ["", "", "", "bcd", "", "", ""]
+
+Note that if +field_sep+ is a \Regexp containing groups,
+their matches are in the returned array, but do not count toward the limit.
+
+If +limit+ is negative, it behaves the same as if +limit+ was +nil+,
+meaning that there is no limit,
+and trailing empty substrings are included:
+
+  'aaabcdaaa'.split('a', -1) # => ["", "", "", "bcd", "", "", ""]
+
+If a block is given, it is called with each substring:
+
+  'abc def ghi'.split(' ') {|substring| p substring }
+
+Output:
+
+  "abc"
+  "def"
+  "ghi"