DOCS-445 background index building with db level locking in 2.2

Author: Sam Kleinman

source/core/indexes.txt

@@ -392,7 +392,12 @@ following:
 Be aware of the following behaviors with background index
 construction:
 
-- A single :program:`mongod` can only build a single index at a time.
+- A :program:`mongod` instance can only build one background index per
+  database, at a time.
+
+  .. versionchanged:: 2.2
+     Before 2.2, a single :program:`mongod` instance could only build
+     one index at a time. 
 
 - The indexing operation runs in the background so that other database
   operations can run while creating the index. However, the