[rubygems/rubygems] 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/rubygems/rubygems/commit/34e8c1ee06

lib/rubygems/compatibility.rb

@@ -1,5 +1,4 @@
 # frozen_string_literal: true
-# :stopdoc:
 
 #--
 # This file contains all sorts of little compatibility hacks that we've
@@ -8,10 +7,13 @@
 #
 # Ruby 1.9.x has introduced some things that are awkward, and we need to
 # support them, so we define some constants to use later.
+#
+# TODO remove at RubyGems 4
 #++
 
-# TODO remove at RubyGems 4
 module Gem
+  # :stopdoc:
+
   RubyGemsVersion = VERSION
   deprecate_constant(:RubyGemsVersion)