update doc.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@28012 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/io/console/console.c

@@ -251,6 +251,8 @@ console_raw(VALUE io)
  *   io.raw!
  *
  * Enables raw mode.
+ *
+ * If the terminal mode needs to be back, use io.raw { ... }.
  */
 static VALUE
 console_set_raw(VALUE io)