Added a note regarding interaction between GitHub and Trac Plugin

docs/internals/contributing/committing-code.txt

@@ -187,7 +187,15 @@ Django's Git repository:
 
   For the curious, we're using a `Trac plugin`_ for this.
 
-  .. _Trac plugin: https://github.com/aaugustin/trac-github
+.. note::
+
+    Note that the Trac integration doesn't know anything about pull requests.
+    So if you try to close a pull request with the phrase "closes #400" in your
+    commit message, GitHub will close the pull request, but the Trac plugin
+    will also close the same numbered ticket in Trac.
+
+
+.. _Trac plugin: https://github.com/aaugustin/trac-github
 
 * If your commit references a ticket in the Django `ticket tracker`_ but
   does *not* close the ticket, include the phrase "Refs #xxxxx", where "xxxxx"