minor clarification of sharded query routing

Author: Sam Kleinman

source/core/sharding.txt

@@ -487,7 +487,8 @@ To route a query to a :term:`cluster <sharded cluster>`,
    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.
+   must direct the query to *all* shards that hold documents for that
+   collection.
 
    .. example::