docs: borg serve --repo is not supported, #8591

src/borg/archiver/serve_cmd.py

@@ -33,6 +33,10 @@ class ServeMixIn:
         - Getting started by some other means (not by the borg client) as a long-running socket
           server to be used for borg clients using a socket://... repository (see the `--socket`
           option if you do not want to use the default path for the socket and pid file).
+
+        Please note that `borg serve` does not support giving a specific repository via the
+        `--repo` option or `BORG_REPO` environment variable. It is always the borg client which
+        specifies the repo to use when talking to `borg serve`.
         """
         )
         subparser = subparsers.add_parser(