DOCS-984 multiple-index builds

source/core/indexes.txt

@@ -557,9 +557,13 @@ following:
 Be aware of the following behaviors with background index
 construction:
 
-- A :program:`mongod` instance can only build one background index per
-  database, at a time.
+- A :program:`mongod` instance can build more than one index in the
+  background concurrently.
 
+  .. versionchanged:: 2.4
+     Before 2.4, a :program:`mongod` instance could 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.

source/release-notes/2.4.txt

@@ -1382,3 +1382,11 @@ and :setting:`setParameter` in the configuration file. Currently
 
 See :doc:`/reference/parameters` for full documentation of available
 parameters and their use.
+
+Multiple Index Builds
+~~~~~~~~~~~~~~~~~~~~~
+
+A single :program:`mongod` instance can build more than one index in
+the background concurrently. This feature is useful when :ref:`building
+indexes in the background <index-creation-background>`. Foreground
+index builds hold a database lock and must proceed one at a time.