DOCSP 21633 adding DISTINCT_SCAN explanation (#1405)

* DOCSP-21633 adding distinct_scan documentation

* DOCSP-21633 fixing bullets

* DOCSP-21633 fixing format

Author: ltran-mdb

source/core/aggregation-pipeline-optimization.txt

@@ -509,8 +509,14 @@ The following list shows some pipeline stages that can use indexes:
   must be the first stage in an aggregation pipeline.
 
 Starting in MongoDB 4.2, in some cases, an aggregation pipeline can use
-a ``DISTINCT_SCAN`` index plan, which typically has higher performance
-than ``IXSCAN``.
+a ``DISTINCT_SCAN`` index plan that returns one document per index key 
+value.
+
+.. note::
+   ``DISTINCT_SCAN`` executes faster than ``IXSCAN`` if multiple 
+   documents per index value exist. However, index scan parameters
+   might affect the time comparison of ``DISTINCT_SCAN`` and
+   ``IXSCAN``. 
 
 Indexes can :ref:`cover <read-operations-covered-query>` queries in an
 aggregation pipeline. A covered query uses an index to return all of the