DOCSP-23351 BACKPORT (#73)

Author: Dave Cuthbert

source/reference/api/reverse.txt

@@ -39,8 +39,13 @@ To use the ``reverse`` endpoint:
 - You must have started your original sync operation with the
   ``reversible`` and ``enableUserWriteBlocking`` options set to
   ``true``.
-
 - ``mongosync`` must be in the ``COMMITTED`` state.
+- Source and destination clusters must be MongoDB 6.0 or later.
+- :ref:`Unique indexes <index-type-unique>` on the original source
+  cluster must be formatted properly. If an upgraded cluster has unique
+  indexes that were created in MongoDB 4.2 or earlier, you must
+  :ref:`resync <resync-replica-member>` all of the nodes in the
+  original source cluster before reversing.
 
 Request
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