(DOCS-11033): Add limitation for 3.2 and earlier multikey indexes

Author: jeff-allen-mongo

source/core/index-multikey.txt

@@ -207,6 +207,20 @@ $expr
 
 :query:`$expr` does not support multikey indexes.
 
+Indexes Built on MongoDB 3.2 or Earlier
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Indexes built on MongoDB 3.2 or earlier do not contain the necessary
+flags to support optimized multikey index use. To benefit from the
+performance enhancements of multikey indexes, you must either:
+
+- Rebuild the older-format indexes on MongoDB 3.4 or later. See
+  :method:`db.collection.reIndex()`.
+
+- On a replica set, :doc:`resync the replica set members
+  </tutorial/resync-replica-set-member/>` containing older-format
+  indexes.
+
 Examples
 --------