DOCS-1563 adding note about background index construction on secondaries

source/core/index-creation.txt

@@ -115,22 +115,28 @@ application code, during designated maintenance windows.
 Building Indexes on Secondaries
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
+.. versionchanged:: 2.5.0
+   Secondary members can now build indexes in the
+   background. Previously all index builds on secondaries were in the
+   foreground.
+
 Background index operations on a :term:`replica set`
-:term:`primary` become foreground indexing operations on
-:term:`secondary members <secondary>` of the set. All indexing
-operations on secondaries block replication.
+:term:`secondaries <secondary>` begin after the :term:`primary`
+completes building the index. If MongoDB builds an index in the
+background on the primary, the secondaries will then build that index
+in the background.
 
 To build large indexes on secondaries the best approach is to
-restart one secondary at a time in :term:`standalone` mode and build the
-index. After building the index, restart as a member of the
-replica set, allow it to catch up with the other members of the
-set, and then build the index on the next secondary. When all the
-secondaries have the new index, step down the primary, restart it
-as a standalone, and build the index on the former primary.
-
-Remember, the amount of time required to build the index on a
-secondary must be within the window of the :term:`oplog`, so
-that the secondary can catch up with the primary.
+restart one secondary at a time in :term:`standalone` mode and build
+the index. After building the index, restart as a member of the
+replica set, allow it to catch up with the other members of the set,
+and then build the index on the next secondary. When all the
+secondaries have the new index, step down the primary, restart it as a
+standalone, and build the index on the former primary.
+
+The amount of time required to build the index on a secondary must be
+within the window of the :term:`oplog`, so that the secondary can
+catch up with the primary.
 
 Indexes on secondary members in "recovering" mode are always built in
 the foreground to allow them to catch up as soon as possible.