string.c: [DOC] add example code for String#strip!

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65068 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

string.c

@@ -8981,6 +8981,9 @@ rb_str_rstrip(VALUE str)
  *  <code>nil</code> if <i>str</i> was not altered.
  *
  *  Refer to <code>strip</code> for the definition of whitespace.
+ *
+ *     "  hello  ".strip!  #=> "hello"
+ *     "hello".strip!      #=> nil
  */
 
 static VALUE