DOCS-420 tcmalloc release notes change

Author: Sam Kleinman

source/release-notes/2.2.txt

@@ -499,6 +499,16 @@ libraries, use the following arguments to ``scons``:
 See the :issue:`SERVER-3829` and :issue:`SERVER-5172` issues for more
 information.
 
+Memory Allocator Changed to TCMalloc
+````````````````````````````````````
+
+To improve performance, In 2.2 MongoDB uses the TCMalloc memory
+allocator from Google Perftools. For more information about this
+change see the :issue:`SERVER-188` and :issue:`SERVER-4683`. For more
+information about TCMalloc, see the documentation of `TCMalloc`_ itself.
+
+.. _`TCMalloc`: http://goog-perftools.sourceforge.net/doc/tcmalloc.html
+
 Replication
 ~~~~~~~~~~~