Do not :stopdoc: entire classes/modules

It disables cross-references to the classes/modules, even if those
are defined in other places.  I suspect this is not an intentional
behavior, however make a workaround for the time being.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65380 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

prelude.rb

@@ -132,8 +132,8 @@ class IO
   end
 end
 
-# :stopdoc:
 class Binding
+  # :nodoc:
   def irb
     require 'irb'
     irb