(DOCS-15662): Remove mention of fullDocumentBeforeChange parameter (#1946)

Author: jeff-allen-mongo

source/reference/operator/aggregation/changeStream.txt

@@ -24,7 +24,6 @@ Definition
         $changeStream: {
           allChangesForCluster: <boolean>,
           fullDocument: <string>,
-          fullDocumentBeforeChange: <string>,
           resumeAfter: <int>
           startAfter: <document>
           startAtOperationTime: <timestamp>
@@ -55,16 +54,6 @@ Definition
           In the case of partial updates, the change notification also
           provides a description of the change.  
 
-      * - ``fullDocumentBeforeChange``
-        - Include the full document from before the change.
-          This field accepts the following values:
-
-          * ``off``: Disables inclusion of the document from before the change.
-          * ``whenAvailable``: Includes document from before the change.
-            The query does not fail if the unmodified document is not available.
-          * ``required``: Includes document from before the change. The query
-            fails if the unmodified document is not available.
-
       * - ``resumeAfter``
         - Specifies a resume token as the logical starting point for 
           the change stream. Cannot be used with ``startAfter`` or
@@ -118,4 +107,3 @@ event notification.  For example:
 
 For more information on change stream notifications, see :ref:`Change Events
 <change-events>`
-