Explain multiple tarball import with ignore-zeros

src/borg/archiver/tar_cmds.py

@@ -445,6 +445,9 @@ class TarMixIn:
         - UNIX V7 tar
         - SunOS tar with extended attributes
 
+        To import multiple tarballs into a single archive, they can be simply
+        concatenated (e.g. using "cat") into a single file, and imported with an
+        ``--ignore-zeros`` option to skip through the stop markers between them.
         """
         )
         subparser = subparsers.add_parser(