Add missing info for Mutex_m to the doc [Bug #13598]

* lib/mutex_m.rb: The #initialize method in a class that includes
  Mutex_m needs calling #super.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65270 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/mutex_m.rb

@@ -25,10 +25,14 @@
 #   obj.extend Mutex_m
 #
 # Or mixin Mutex_m into your module to your class inherit Mutex instance
-# methods.
+# methods ---remember to call super() in your class initialize method.
 #
 #   class Foo
 #     include Mutex_m
+#     def initialize ...
+#       ...
+#       super()
+#     end
 #     # ...
 #   end
 #   obj = Foo.new