create: document exclusion through nodump

(cherry picked from commit 495f838d88)

src/borg/archiver.py

@@ -2807,6 +2807,9 @@ class Archiver:
         and not include any other contents of the containing folder, this can be enabled
         through using the ``--keep-exclude-tags`` option.
 
+        Borg respects the nodump flag. Files flagged nodump will be marked as excluded (x)
+        in ``--list`` output.
+
         Item flags
         ++++++++++