markdown: Clarify stream name recommendations in create-stream.

Now, we further clarify in the create-stream.md macro that even
if users follow our default stream name recommendation for a
given integration, said stream must still be explicitly created.

templates/zerver/help/include/create-stream.md

@@ -1,4 +1,5 @@
 First, create the stream you'd like to use for
 {{ integration_display_name }} notifications, and subscribe all
 interested parties to this stream. We recommend the
-name `{{ recommended_stream_name }}`.
+name `{{ recommended_stream_name }}`. You still need to create
+the stream even if you are using this default recommendation.