create: document exclusion through nodump

(cherry picked from commit 495f838d88dea02ba3ec08f5985a4827c37a76d1)

borg/archiver.py

@@ -1392,6 +1392,9 @@ class Archiver:
 
         See the output of the "borg help patterns" command for more help on exclude patterns.
         See the output of the "borg help placeholders" command for more help on placeholders.
+
+        Borg respects the nodump flag. Files flagged nodump will be marked as excluded (x)
+        in ``--list`` output.
         """)
 
         subparser = subparsers.add_parser('create', parents=[common_parser],