DOCS-9801 advise backing up FTDC data before resync

Author: andf-mongodb

source/tutorial/resync-replica-set-member.txt

@@ -78,10 +78,16 @@ perform an initial sync by restarting the instance without the contents of the
    method from the :binary:`~bin.mongo` shell or on Linux systems, the
    :option:`mongod --shutdown` option.
 
-2. Delete all data and sub-directories from the member's
-   :setting:`~storage.dbPath` directory. Consider making a backup first.
+2. (Optional) Make a backup of all data and sub-directories from the
+   member's :setting:`~storage.dbPath` directory. If a full backup is
+   not required, consider backing up just the ``diagnostic.data``
+   directory to preserve potentially-useful troubleshooting data in the
+   event of an issue. See :ref:`ftdc-stub` for more information.
 
-3. :doc:`Start the mongod process </tutorial/manage-mongodb-processes>`,
+3. Delete all data and sub-directories from the member's
+   :setting:`~storage.dbPath` directory.
+
+4. :doc:`Start the mongod process </tutorial/manage-mongodb-processes>`,
    specifying :ref:`cli-mongod-replica-set` as appropriate.
 
 At this point, the :binary:`~bin.mongod` performs an initial sync. The length of