DOCS-4850: Blurb to mention that parallelCollectionScan is not accepted by mongos

Author: i80and

source/reference/command/parallelCollectionScan.txt

@@ -35,6 +35,9 @@ parallelCollectionScan
 
    .. include:: /includes/apiargs/dbcommand-parallelCollectionScan-field.rst
 
+   :dbcommand:`parallelCollectionScan` is only offered by :program:`mongod`,
+   and cannot operate on a sharded cluster.
+
 Output
 ------