[DOC] Tweaks for String#downcase

doc/string/downcase.rdoc

@@ -0,0 +1,12 @@
+Returns a new string containing the downcased characters in +self+:
+
+   'Hello, World!'.downcase      # => "hello, world!"
+   'ТЕСТ'.downcase               # => "тест"
+   'よろしくお願いします'.downcase # => "よろしくお願いします"
+
+Some characters do not have upcased and downcased versions.
+
+The casing may be affected by the given +mapping+;
+see {Case Mapping}[rdoc-ref:case_mapping.rdoc].
+
+Related: see {Converting to New String}[rdoc-ref:String@Converting+to+New+String].

string.c

@@ -8181,15 +8181,7 @@ rb_str_downcase_bang(int argc, VALUE *argv, VALUE str)
  *  call-seq:
  *    downcase(mapping) -> string
  *
- *  Returns a string containing the downcased characters in +self+:
- *
- *     s = 'Hello World!' # => "Hello World!"
- *     s.downcase         # => "hello world!"
- *
- *  The casing may be affected by the given +mapping+;
- *  see {Case Mapping}[rdoc-ref:case_mapping.rdoc].
- *
- *  Related: String#downcase!, String#upcase, String#upcase!.
+ *  :include: doc/string/downcase.rdoc
  *
  */