(DOCSP-25926): Clarify mongos index use (#2028)

* (DOCSP-25926): Clarify mongos index use

* wording

* wording

* minimalism

Author: jeff-allen-mongo

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

@@ -313,16 +313,10 @@ operation<sharding-mongos-broadcast>` to fulfill these queries.
 Index Use
 ~~~~~~~~~
 
-If the query does not include the :term:`shard key`, the
-:binary:`~bin.mongos` must send the query to all shards as a
-"scatter/gather" operation. Each shard will, in turn, use *either* the
-shard key index or another more efficient index to fulfill the query.
-
-If the query includes multiple sub-expressions that reference the
-fields indexed by the shard key *and* the secondary index, the
-:binary:`~bin.mongos` can route the queries to a specific shard and the
-shard will use the index that will allow it to fulfill most
-efficiently.
+When a shard receives a query, it uses the most efficient index
+available to fulfill that query. The index used may be either the
+:ref:`shard key index <sharding-shard-key-indexes>` or another eligible
+index present on the shard.
 
 Sharded Cluster Security
 ------------------------