DOCSP-15110 added information about postBatchResumeToken (#97)

* added information on postBatchResumeToken

Author: kyuan-mongodb

source/kafka-source.txt

@@ -482,10 +482,13 @@ an example source connector configuration file, see
      - int
 
      - | The length of time in milliseconds between sending heartbeat
-         messages. Heartbeat messages contain the post batch resume token
-         and are sent when no source records have been published in the
-         specified interval. This improves the resumability of the
-         connector for low volume namespaces. Use ``0`` to disable.
+         messages. Heartbeat messages contain a
+         ``postBatchResumeToken``. The value of this field contains the
+         MongoDB server oplog entry that was most recently read from the
+         change stream. The connector sends heartbeat messages when
+         source records are not published in the specified interval.
+         This improves the resumability of the connector for low volume
+         namespaces. Use ``0`` to disable. 
        | **Default**: ``0``
        | **Accepted Values**: An integer