Add a note about updating phpdoc in a patch release

Author: fabpot

contributing/code/maintenance.rst

@@ -94,6 +94,9 @@ patch version:
 * **Web design**: Changing the web design of built-in pages like exceptions,
   the toolbar or the profiler is not allowed.
 
+* **Adding or updating annotations**: Adding or updating annotations (PHPDoc
+  annotations for instance) is not allowed.
+
 .. note::
 
     This policy is designed to enable a continuous upgrade path that allows one