DOCS-290 adding indexCounters to release notes

Author: Sam Kleinman

source/release-notes/2.2.txt

@@ -387,6 +387,13 @@ Documentation: :ref:`sharding-shard-key-indexes`
 Additional Improvements
 ~~~~~~~~~~~~~~~~~~~~~~~
 
+``indexCounters`` No Longer Report Sampled Data
+```````````````````````````````````````````````
+
+:status:`indexCounters` now report actual counters that reflect index
+use and state. In previous versions, these data were sampled. See
+:issue:`SERVER-5784` and :status:`indexCounters` for more information.
+
 ``_id`` Index not Required for Updates to Capped Collections
 ````````````````````````````````````````````````````````````