* string.c: [DOC] Specify to_s on subclasses [ci skip]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49441 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

string.c

@@ -4877,7 +4877,9 @@ rb_str_to_f(VALUE str)
  *     str.to_s     -> str
  *     str.to_str   -> str
  *
- *  Returns the receiver.
+ *  Returns +self+.
+ *
+ *  If called on a subclass of String, converts the receiver to a String object.
  */
 
 static VALUE