DOCSP-13944 clarify _id resumeToken definition for change streams

source/reference/change-events.txt

@@ -62,8 +62,10 @@ following table describes each field in the change stream response document:
      - document
      - .. _change-stream-event-id:
 
-       Metadata related to the operation. Acts as the ``resumeToken``
+       A :term:`BSON` object which serves as an identifier for the
+       change stream event. This value is used as the ``resumeToken``
        for the ``resumeAfter`` parameter when resuming a change stream.
+       The ``_id`` object has the following form:
 
        .. code-block:: none
 
@@ -72,9 +74,13 @@ following table describes each field in the change stream response document:
           }
 
        The ``_data`` type depends on the MongoDB versions
-       and, in some cases, the feature compatibility version (fcv) at
-       the time of the change stream's opening/resumption. For details,
-       see :ref:`change-stream-resume-token`.
+       and, in some cases, the :ref:`feature compatibility version (fCV)
+       <view-fcv>` at the time of the change stream's
+       opening/resumption. See :ref:`change-stream-resume-token` for the
+       full list of ``_data`` types.
+
+       See :ref:`change-stream-resume` for an example of resuming a
+       change stream by ``resumeToken``.
 
    * - ``operationType``
      - string