[ruby/forwardable] Move :stopdoc: directive

If this is at the top level, it stops the documentation of the
entire module, but not only the part in this file.

https://github.com/ruby/forwardable/commit/34cd2ac4a1

lib/forwardable/impl.rb

@@ -1,5 +1,6 @@
-# :stopdoc:
 module Forwardable
+  # :stopdoc:
+
   def self._valid_method?(method)
     catch {|tag|
       eval("BEGIN{throw tag}; ().#{method}", binding, __FILE__, __LINE__)