DOCSP-23551 Adds reversible note

source/reference/api/start.txt

@@ -132,6 +132,9 @@ Request Body Parameters
        original source cluster blocks writes and the destination cluster
        accepts writes.
 
+       To reverse sync, the ``enableUserWriteBlocking`` field must be set
+       to ``true``.
+
        Default value is ``false``.
 
    * - ``includeNamespaces``
@@ -158,14 +161,17 @@ Request Body Parameters
      - If set to ``true``, enables the sync operation to be
        reversed. 
 
+       To reverse sync, the ``enableUserWriteBlocking`` field must be set 
+       to ``true``.      
+
        This option is not supported for the following configurations:
 
        * Sync from a replica set to a sharded cluster
 
        * Sync sharded clusters that have different numbers of shards
 
        * Reversible sync when ``buildIndexes`` is set to ``never``.
-       
+
        For more information, see the :ref:`reverse <c2c-api-reverse>` endpoint.
 
        Default value is ``false``.