DOCS-15512 Adds $merge to db.collection.aggregate return (#2376) (#2412)

* DOCS-15512 Adds  to db.collection.aggregate return

* Fixes per Sarah Olsen

* Fixes per Joe

Author: kennethdyer

source/reference/method/db.collection.aggregate.txt

@@ -209,14 +209,18 @@ Definition
 
 
    :returns:
-      A :term:`cursor` to the documents produced by the final stage of
-      the aggregation pipeline operation, or if you include the
-      ``explain`` option, the document that provides
-      details on the processing of the aggregation operation.
-
-      If the pipeline includes the :pipeline:`$out` operator,
-      :method:`~db.collection.aggregate()` returns an empty cursor. See
-      :pipeline:`$out` for more information.
+      This method returns:
+
+      - A :term:`cursor` for the documents produced by the final stage of
+        the aggregation pipeline.
+
+      - If the pipeline includes the ``explain`` option, the query returns 
+        a document that provides details on the processing of the 
+        aggregation operation.
+
+      - If the pipeline includes the :pipeline:`$out` or
+        :pipeline:`$merge` operators, the query returns an empty cursor.
+
 
 Behavior
 --------