Updating documentation for app.after_request decorator.

Signed-off-by: Armin Ronacher <armin.ronacher@active-4.com>

flask/app.py

@@ -713,7 +713,10 @@ class Flask(_PackageBoundObject):
         return f
 
     def after_request(self, f):
-        """Register a function to be run after each request."""
+        """Register a function to be run after each request. Your function
+        must take one parameter, a :attr:`response_class` object and return
+        a new response object or the same (see :meth:`process_response`).
+        """
         self.after_request_funcs.setdefault(None, []).append(f)
         return f