(DOCS-13197): Explain result improvements for mongos

Author: jeff-allen-mongo

source/reference/explain-results.txt

@@ -637,7 +637,8 @@ the ``explain`` operation.
            content: |
 
               For sharded collections, ``explain`` returns the
-              ``serverInfo`` for each accessed shard.
+              ``serverInfo`` for each accessed shard, and a top-level
+              ``serverInfo`` object for the :binary:`~bin.mongos`.
 
               .. code-block:: none
 
@@ -659,6 +660,14 @@ the ``explain`` operation.
                           }
                           ...
                        ]
+                    }
+                 },
+                 "serverInfo" : {      // serverInfo for mongos
+                   "host" : <string>,
+                   "port" : <int>,
+                   "version" : <string>,
+                   "gitVersion" : <string>
+                 }
 
 3.0 Format Change
 -----------------

source/release-notes/4.2.txt

@@ -1794,6 +1794,24 @@ Starting in MongoDB 4.2, the ``explain`` output can include a new
 :data:`~explain.queryPlanner.optimizedPipeline` field. For details,
 refer to :data:`~explain.queryPlanner.optimizedPipeline`.
 
+Starting in version 4.2 (and 4.0.14, 3.6.16):
+
+- :doc:`Explain results </reference/explain-results/>` for commands run
+  on sharded clusters include a top-level :ref:`serverInfo <serverInfo>`
+  object for the :binary:`~bin.mongos` in addition to the ``serverInfo``
+  objects returned for each shard.
+
+- :doc:`Explain results </reference/explain-results/>` include the
+  :ref:`serverInfo <serverInfo>` object when
+  :data:`~explain.queryPlanner.optimizedPipeline` is ``true``. In
+  previous versions of MongoDB, ``explain`` results would occasionally
+  not include the ``serverInfo`` object when
+  :data:`~explain.queryPlanner.optimizedPipeline` was ``true``.
+
+.. seealso::
+
+   :doc:`Explain results </reference/explain-results/>`.
+
 Change to ``isMaster`` Output
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~