Adds new "specificShard" info and changes formatting (#5251)

* replacing PR 5229

* review changes

* final review change

Author: MongoCaleb

source/core/sharded-cluster-query-router.txt

@@ -70,10 +70,18 @@ randomly-selected shard in the set of shards targeted by the aggregation.
 For more information on how the work of aggregation is split among
 components of a sharded cluster query, use ``explain:true`` as a
 parameter to the :method:`~db.collection.aggregate()` call. The
-return includes three json objects. ``mergeType`` shows where the
-stage of the merge happens ("primaryShard", "anyShard", or "mongos").
-``splitPipeline`` shows which operations in your pipeline have run on
-individual shards. ``shards`` shows the work each shard has done.
+return includes three JSON objects: 
+
+- ``mergeType`` shows where the
+  stage of the merge happens ("primaryShard", "anyShard", "specificShard", 
+  or "mongos"). When ``mergeType`` is ``specificShard``, the aggregate 
+  output includes a ``mergeShard`` property that contains the shard ID of the 
+  merging shard. 
+
+- ``splitPipeline`` shows which operations in your pipeline have 
+  run on individual shards. 
+
+- ``shards`` shows the work each shard has done.
 
 In some cases, when the :term:`shard key` or a prefix of the shard key
 is a part of the query, the :binary:`~bin.mongos` performs a