DOCS-334 migration threshold release notes

Author: Sam Kleinman

source/release-notes/2.2.txt

@@ -444,6 +444,15 @@ Index on Shard Keys Can Now Be a Compound Index
 
 Documentation: :ref:`sharding-shard-key-indexes`
 
+Migration Thresholds Modified
+`````````````````````````````
+
+The :ref:`migration thresholds <sharding-migration-thresholds>` have
+changed in 2.2 to permit more even distribution of :term:`chunks
+<chunk>` in collections with smaller quantities of data. See the
+:ref:`sharding-migration-thresholds` documentation for more
+information.
+
 Additional Improvements
 ~~~~~~~~~~~~~~~~~~~~~~~