DOCSP-26745 clarify sortby limitation (#6816)

* DOCSP-26745 clarify sortby limitation

* DOCSP-26745 clarify sortby limitation

Author: ltran-mdb2

source/reference/operator/aggregation/rank.txt

@@ -21,10 +21,12 @@ Returns the document position (known as the rank) relative to other
 documents in the :pipeline:`$setWindowFields` stage :ref:`partition
 <setWindowFields-partitionBy>`.
 
-The :pipeline:`$setWindowFields` stage :ref:`sortBy
-<setWindowFields-sortBy>` field value determines the document rank. For
-more information on how MongoDB compares fields with different types,
-see :ref:`BSON comparison order <bson-types-comparison-order>`.
+The :ref:`sortBy <setWindowFields-sortBy>` field value in the
+:pipeline:`$setWindowFields` stage determines the document rank. When
+used with the ``$rank`` operator, ``sortBy`` can only take one field as
+its value. For more information on how MongoDB compares fields with
+different types, see :ref:`BSON comparison order
+<bson-types-comparison-order>`.
 
 If multiple documents occupy the same rank, :group:`$rank` places
 the document with the subsequent value at a rank with a gap