Use "agg-out" reference in MongoDB manual

See: https://github.com/mongodb/docs/commit/2a912319800d932b5fd5dc2dfee3d591e4311b9f

docs/includes/apiargs-MongoDBCollection-method-aggregate-option.yaml

@@ -29,7 +29,7 @@ source:
   file: apiargs-MongoDBCollection-common-option.yaml
   ref: bypassDocumentValidation
 post: |
-  This only applies when using the :pipeline:`$out` stage.
+  This only applies when using the :ref:`$out <agg-out>` stage.
 
   Document validation requires MongoDB 3.2 or later: if you are using an earlier
   version of MongoDB, this option will be ignored.
@@ -75,7 +75,7 @@ source:
   file: apiargs-MongoDBCollection-common-option.yaml
   ref: writeConcern
 post: |
-  This only applies when the ``$out`` stage is specified.
+  This only applies when the :ref:`$out <agg-out>` stage is specified.
 
   This is not supported for server versions prior to 3.4 and will result in an
   exception at execution time if used.