Improve description of aggregation $count

Re: Automattic/mongoose#7114, the current description is misleading because it implies that if `$count` is the last aggregation stage, you should get a single document as output rather than a cursor or array.

Author: vkarpov15

source/reference/operator/aggregation/count.txt

@@ -17,8 +17,8 @@ Definition
 
    .. versionadded:: 3.4
 
-   Returns a document that contains a count of the number of documents
-   input to the stage.
+   Passes a document to the next stage that contains a count of the
+   number of documents input to the stage.
 
    :pipeline:`$count` has the following prototype form: