added reversing descr (#355) (#357)

Author: sonderdonk-mdb

source/reference/api/reverse.txt

@@ -135,7 +135,10 @@ Response
 Behavior
 --------
 
-If the ``reverse`` request is successful, ``mongosync`` enters the
+The reverse endpoint starts the ``REVERSING`` state. ``mongosync`` swaps the 
+source and destination clusters and resumes applying change events.
+
+If the ``reverse`` sync is successful, ``mongosync`` enters the
 ``RUNNING`` state. The synchronization continues in the reverse
 direction from the original sync job. You do not need to restart the
 entire sync process to copy the original data.