Small documentation adjustments for new/updated features (#15634)

* Document Range#to_set

* Update Thread#raise and Fiber#raise signatures and docs

* Add reference to String#strip to character_selectors.rdoc

* Update *nil docs when calling methods

* Enhance Array#find and #rfind docs

* Add a notice to Kernel#raise about cause:

1 files changed, 4 insertions, 8 deletions

diff --git a/array.c b/array.c
index 27b84249bf..e13239ad3d 100644
--- a/array.c
+++ b/array.c
@@ -2102,9 +2102,7 @@ rb_ary_fetch(int argc, VALUE *argv, VALUE ary)
  *
  * If no such element is found, calls +if_none_proc+ and returns its return value.
  *
- *   [1, 3, 5].find(proc {false}) {|element| element > 12} # => false
- *   [[:foo, 0], [:bar, 1], [:baz, 2]].find {|key, value| key.start_with?('b') }            # => [:bar, 1]
- *   [[:foo, 0], [:bar, 1], [:baz, 2]].find(proc {[]}) {|key, value| key.start_with?('c') } # => []
+ *   [1, 3, 5].find(proc {-1}) {|element| element > 12} # => -1
  *
  * With no block given, returns an Enumerator.
  *
@@ -2140,16 +2138,14 @@ rb_ary_find(int argc, VALUE *argv, VALUE ary)
  * Returns the last element for which the block returns a truthy value.
  *
  * With a block given, calls the block with successive elements of the array in
- * reverse order; returns the last element for which the block returns a truthy
+ * reverse order; returns the first element for which the block returns a truthy
  * value:
  *
- *   (0..9).rfind {|element| element < 5}                # => 4
+ *   [1, 2, 3, 4, 5, 6].rfind {|element| element < 5}       # => 4
  *
  * If no such element is found, calls +if_none_proc+ and returns its return value.
  *
- *   (0..9).rfind(proc {false}) {|element| element < -2} # => false
- *   {foo: 0, bar: 1, baz: 2}.rfind {|key, value| key.start_with?('b') }            # => [:baz, 2]
- *   {foo: 0, bar: 1, baz: 2}.rfind(proc {[]}) {|key, value| key.start_with?('c') } # => []
+ *   [1, 2, 3, 4].rfind(proc {0}) {|element| element < -2}  # => 0
  *
  * With no block given, returns an Enumerator.
  *