Added documentation for attach_extend()

lib/class.js

@@ -160,6 +160,13 @@ exports.extend = function( base )
 }
 
 
+/**
+ * Attaches extend method to the given function's prototype
+ *
+ * @param  {Function}  func  function to attach method to
+ *
+ * @return undefined
+ */
 var attach_extend = function( func )
 {
     /**