DOCSP-21633 v4.2 backport(#1414)

Author: ltran-mdb

source/core/aggregation-pipeline-optimization.txt

@@ -387,8 +387,18 @@ Indexes
 -------
 
 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
+documents and has high performance.
 
 Example
 -------