DOCS-15881 m to n sharding (#109)

Author: Dave Cuthbert

source/includes/fact-reverse-limitation.rst

@@ -0,0 +1,3 @@
+The source and destination clusters must have the same number of
+shards. You cannot reverse sync when the clusters have different
+topologies.

source/multiple-mongosyncs.txt

@@ -52,8 +52,10 @@ To configure multiple ``mongosync`` instances:
 
       .. _c2c-shard-config-verify:
 
-      The source cluster and destination cluster must have the same
-      number of shards.
+      Starting in ``mongosync`` 1.1, you can sync between clusters with
+      different numbers of shards. However, if you want to reverse the
+      sync, the source cluster and destination cluster must have the
+      same number of shards.
 
    .. step:: Determine the shard IDs
 

source/reference/api/reverse.txt

@@ -46,6 +46,7 @@ To use the ``reverse`` endpoint:
   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.
+- .. include:: /includes/fact-reverse-limitation.rst
 
 .. include:: /includes/fact-reverse-sync-action-types.rst
 

source/reference/limitations.txt

@@ -115,9 +115,10 @@ Sharded Clusters
 Reversing
 ---------
 
-If the old source has :ref:`unique indexes <index-type-unique>` which
-are partially distributed across shards, reversing may cause failures.
-Ensure that unique indexes exist on all shards before reversing.
+- If the old source has :ref:`unique indexes <index-type-unique>` which
+  are partially distributed across shards, reversing may cause failures.
+  Ensure that unique indexes exist on all shards before reversing.
+- .. include:: /includes/fact-reverse-limitation.rst
 
 Multiple Clusters
 -----------------