DOCSP-25519 uniqueness of id (#94)

* DOCSP-25519 unique _id field

* DOCSP-25519 unique _id field

Author: Dave Cuthbert

source/reference/limitations.txt

@@ -80,6 +80,9 @@ Sharded Clusters
   - Replica set to sharded cluster.
   - Sharded cluster to replica set.
   - Unequal numbers of source and destination shards.
+- Within a collection, the ``_id`` field must be unique across all of
+  the shards in the cluster. See :ref:`sharded-clusters-unique-indexes`
+  for more details.
 - The :dbcommand:`movePrimary` command cannot be used to reassign the
   primary shard while syncing.
 - There is no replication for zone configuration. ``mongosync``