Mention register_error_handler in errorhandler doc

The documentation for @errorhandler gives "app.error_handler_spec[None][404] = page_not_found" as an example for adding an error handler without the decorator.  However, register_error_handler appears to be the correct way to do this (added 0.7), and it eliminates the problems with modifying error_handler_spec directly.

flask/app.py

@@ -1065,6 +1065,11 @@ class Flask(_PackageBoundObject):
         The first `None` refers to the active blueprint.  If the error
         handler should be application wide `None` shall be used.
 
+        .. versionadded:: 0.7
+            Use :meth:`register_error_handler` instead of modifying
+            :attr:`error_handler_spec` directly, for application wide error
+            handlers.
+            
         .. versionadded:: 0.7
            One can now additionally also register custom exception types
            that do not necessarily have to be a subclass of the