(DOCSP-42593) Add info about necessity of ns field.

Author: lmkerbey-mdb

source/source-connector/usage-examples/custom-pipeline.txt

@@ -59,10 +59,12 @@ to your ``pipeline`` setting:
 
 .. important::
 
-   Make sure that the results of the pipeline contain the top-level ``_id``
-   field of the ``payload`` object, which MongoDB uses as the value of the
-   :manual:`resume token </changeStreams/#resume-a-change-stream>`.
-
+   Make sure that the results of the pipeline contain the top-level
+   ``_id`` and ``ns`` fields of the ``payload`` object. MongoDB uses
+   ``id`` as the value of the :manual:`resume token
+   </changeStreams/#resume-a-change-stream>` and ``ns`` to generate the
+   Kafka output topic name.
+   
 When the application inserts the sample document, your configured
 connector publishes the following record to your Kafka topic: