Add clarification for dot notation to densify

Author: jeff-allen-mongo

source/reference/operator/aggregation/densify.txt

@@ -66,6 +66,8 @@ The :pipeline:`$densify` stage takes a document with these fields:
        Documents that do not contain the specified ``field`` continue
        through the pipeline unmodified.
 
+       .. include:: /includes/use-dot-notation.rst
+
        For restrictions, see :ref:`densify-field-restrictions`.
 
    * - :ref:`partitionByFields <densify-partition-by-fields>`