release-notes: adding reference to release notes in findandmodify document.

Author: Sam Kleinman

source/reference/commands.txt

@@ -517,8 +517,13 @@ Aggregation
 
    :field upsert: when ``true``, creates a new document if the
                   specified ``query`` returns no documents. The
-                  default is ``false``.
+                  default is ``false``. The return value for these
+                  operations is ``null`` in version 2.2
 
+   .. versionchanged:: 2.2
+      Previously, ``upsert`` operations returned a an empty document
+      (e.g. ``{ }``,) see :ref:`the 2.2 release notes
+      <2.2-findandmodify-returns-null>` for more information.
 
    For example:
 

source/release-notes/2.2.txt

@@ -151,6 +151,8 @@ Compatibility Changes
 .. TODO add sections here to describe compatibility changes with
    regards to authentication, drivers, and mongos'
 
+.. _2.2-findandmodify-returns-null:
+
 ``findAndMondify`` Returns Null value for Upserts
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