(DOCSP-45308) [c2c] add note on how to verify shard keys (#526) (#529)

* (DOCSP-45308) Edits.

* (DOCSP-45308) Verify shard key.

* (DOCSP-45308) Edits.

* (DOCSP-45308) Condense code example.

* (DOCSP-45308) edits.

* (DOCSP-45308) Edits.

* Update source/reference/verification.txt



* Update source/reference/verification.txt



---------

Co-authored-by: Ashley Brown <[email protected]>
Co-authored-by: Felipe Gasper <[email protected]>

Author: 3 people

source/reference/verification.txt

@@ -118,6 +118,41 @@ source to the destination cluster.
        :method:`db.getCollectionInfos` method on the source and
        destination clusters and compare the results.
 
+   * - .. _c2c-shardkey-comparison: 
+
+       Shard Key Comparison
+     - To verify the transfer of shard keys to a synced collection, run a query on the ``config.collections``
+       collection to find a document whose ``_id`` value is the namespace of the target collection. 
+       Compare the ``key`` value of this document in the source and destination clusters.
+
+       For example, for a collection named ``pets`` in the ``records`` database, you can verify the shard key
+       using the following query in :binary:`mongosh`: 
+
+       .. io-code-block:: 
+          :copyable: true
+
+          .. input::
+             :language: javascript 
+
+             db.getSiblingDB("config").collections.find({ _id : "records.pets" }) 
+
+          .. output:: 
+             :language: javascript
+             :emphasize-lines: 5-7
+             :visible: false
+
+             {
+                 "_id" : "records.pets",
+                 "lastmod" : ISODate("2021-07-21T15:48:15.193Z"),
+                 "timestamp": Timestamp(1626882495, 1),
+                 "key" : {
+                       "_id" : 1
+                 },
+                 "unique" : false,
+                 "lastmodEpoch" : ObjectId("5078407bd58b175c5c225fdc"),
+                 "uuid" :  UUID("f8669e52-5c1b-4ea2-bbdc-a00189b341da")
+             }
+
    * - :ref:`Migration Verifier <c2c-migration-verifier>`
      - Migration Verifier connects to the source and destination
        clusters and performs a series of verification checks,