DOCS-13229 correct merge behavior sharded cluster

Author: andf-mongodb

source/core/aggregation-pipeline-sharded-collections.txt

@@ -30,9 +30,16 @@ to be done on the primary shard.
 .. versionchanged:: 3.6
 
 When aggregation operations run on multiple shards, the results are 
-routed to the :binary:`~bin.mongos` to be merged. However, :pipeline:`$out`
-and :pipeline:`$lookup` must run on the :ref:`primary shard <primary-shard>` 
-and are aggregated there.  
+routed to the :binary:`~bin.mongos` to be merged, except in the
+following cases:
+
+- If the pipeline includes the :pipeline:`$out` or
+  :pipeline:`$lookup` stages, the merge runs on the
+  :ref:`primary shard <primary-shard>`.
+
+- If the pipeline includes a sorting or grouping stage, and the
+  :ref:`allowDiskUse <cmd-aggregate-allowDiskUse>` setting is enabled,
+  the merge runs on a randomly-selected shard.
 
 Optimization
 ------------