DOCS-443 edit to add shard key prefix example

Author: Sam Kleinman

source/core/sharding.txt

@@ -464,10 +464,28 @@ To route a query to a :term:`cluster <sharded cluster>`,
 
 #. Determine the list of :term:`shards` that must receive the query.
 
-   In some cases, when the :term:`shard key` is a part of the query,
-   the :program:`mongos` can route the query to a subset of the
-   shards. Otherwise, the :program:`mongos` must direct the query to
-   *all* shards.
+   In some cases, when the :term:`shard key` or a prefix of the shard
+   key is a part of the query, the :program:`mongos` can route the
+   query to a subset of the shards. Otherwise, the :program:`mongos`
+   must direct the query to *all* shards.
+
+   .. example::
+
+      Given the following shard key:
+
+      .. code-block:: javascript
+
+         { zipcode: 1, u_id: 1, c_date: 1 }
+
+      Depending on the distribution of chunks in the cluster, the
+      :program:`mongos` may be able to target the query at a subset of
+      shards, if the query contains the following fields:
+
+      .. code-block:: javascript
+
+         { zipcode: 1 }
+         { zipcode: 1, u_id: 1 }
+         { zipcode: 1, u_id: 1, c_date: 1 }
 
 #. Establish a cursor on all targeted shards.