[ruby/stringio] [DOC] Tweaks for StringIO#each_line

(https://github.com/ruby/stringio/pull/165)

Adds to "Position":  pos inside a character.

Makes a couple of minor corrections.

---------

https://github.com/ruby/stringio/commit/ff332abafa

Co-authored-by: Sutou Kouhei <kou@cozmixng.org>

1 files changed, 189 insertions, 0 deletions

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+With a block given calls the block with each remaining line (see "Position" below) in the stream;
+returns `self`.
+
+Leaves stream position at end-of-stream.
+
+**No Arguments**
+
+With no arguments given,
+reads lines using the default record separator
+(global variable `$/`, whose initial value is `"\n"`).
+
+```ruby
+strio = StringIO.new(TEXT)
+strio.each_line {|line| p line }
+strio.eof? # => true
+```
+
+Output:
+
+```
+"First line\n"
+"Second line\n"
+"\n"
+"Fourth line\n"
+"Fifth line\n"
+```
+
+**Argument `sep`**
+
+With only string argument `sep` given,
+reads lines using that string as the record separator:
+
+```ruby
+strio = StringIO.new(TEXT)
+strio.each_line(' ') {|line| p line }
+```
+
+Output:
+
+```
+"First "
+"line\nSecond "
+"line\n\nFourth "
+"line\nFifth "
+"line\n"
+```
+
+**Argument `limit`**
+
+With only integer argument `limit` given,
+reads lines using the default record separator;
+also limits the size (in characters) of each line to the given limit:
+
+```ruby
+strio = StringIO.new(TEXT)
+strio.each_line(10) {|line| p line }
+```
+
+Output:
+
+```
+"First line"
+"\n"
+"Second lin"
+"e\n"
+"\n"
+"Fourth lin"
+"e\n"
+"Fifth line"
+"\n"
+```
+
+**Arguments `sep` and `limit`**
+
+With arguments `sep` and `limit` both given,
+honors both:
+
+```ruby
+strio = StringIO.new(TEXT)
+strio.each_line(' ', 10) {|line| p line }
+```
+
+Output:
+
+```
+"First "
+"line\nSecon"
+"d "
+"line\n\nFour"
+"th "
+"line\nFifth"
+" "
+"line\n"
+```
+
+**Position**
+
+As stated above, method `each` _remaining_ line in the stream.
+
+In the examples above each `strio` object starts with its position at beginning-of-stream;
+but in other cases the position may be anywhere (see StringIO#pos):
+
+```ruby
+strio = StringIO.new(TEXT)
+strio.pos = 30 # Set stream position to character 30.
+strio.each_line {|line| p line }
+```
+
+Output:
+
+```
+" line\n"
+"Fifth line\n"
+```
+
+In all the examples above, the stream position is at the beginning of a character;
+in other cases, that need not be so:
+
+```ruby
+s = 'こんにちは'  # Five 3-byte characters.
+strio = StringIO.new(s)
+strio.pos = 3   # At beginning of second character.
+strio.each_line {|line| p line }
+strio.pos = 4   # At second byte of second character.
+strio.each_line {|line| p line }
+strio.pos = 5   # At third byte of second character.
+strio.each_line {|line| p line }
+```
+
+Output:
+
+```
+"んにちは"
+"\x82\x93にちは"
+"\x93にちは"
+```
+
+**Special Record Separators**
+
+Like some methods in class `IO`, StringIO.each honors two special record separators;
+see {Special Line Separators}[https://docs.ruby-lang.org/en/master/IO.html#class-IO-label-Special+Line+Separator+Values].
+
+```ruby
+strio = StringIO.new(TEXT)
+strio.each_line('') {|line| p line } # Read as paragraphs (separated by blank lines).
+```
+
+Output:
+
+```
+"First line\nSecond line\n\n"
+"Fourth line\nFifth line\n"
+```
+
+```ruby
+strio = StringIO.new(TEXT)
+strio.each_line(nil) {|line| p line } # "Slurp"; read it all.
+```
+
+Output:
+
+```
+"First line\nSecond line\n\nFourth line\nFifth line\n"
+```
+
+**Keyword Argument `chomp`**
+
+With keyword argument `chomp` given as `true` (the default is `false`),
+removes trailing newline (if any) from each line:
+
+```ruby
+strio = StringIO.new(TEXT)
+strio.each_line(chomp: true) {|line| p line }
+```
+
+Output:
+
+```
+"First line"
+"Second line"
+""
+"Fourth line"
+"Fifth line"
+```
+
+With no block given, returns a new {Enumerator}[https://docs.ruby-lang.org/en/master/Enumerator.html].
+
+
+Related: StringIO.each_byte, StringIO.each_char, StringIO.each_codepoint.