docs: serve: env vars in original commands are ignored

docs/usage.rst

@@ -403,11 +403,17 @@ Examples
 borg serve has special support for ssh forced commands (see ``authorized_keys``
 example below): it will detect that you use such a forced command and extract
 the value of the ``--restrict-to-path`` option(s).
+
 It will then parse the original command that came from the client, makes sure
 that it is also ``borg serve`` and enforce path restriction(s) as given by the
 forced command. That way, other options given by the client (like ``--info`` or
 ``--umask``) are preserved (and are not fixed by the forced command).
 
+Environment variables (such as BORG_HOSTNAME_IS_UNIQUE) contained in the original
+command sent by the client are *not* interpreted, but ignored. If BORG_XXX environment
+variables should be set on the ``borg serve`` side, then these must be set in system-specific
+locations like ``/etc/environment`` or in the forced command itself (example below).
+
 ::
 
     # Allow an SSH keypair to only run borg, and only have access to /path/to/repo.
@@ -416,6 +422,9 @@ forced command. That way, other options given by the client (like ``--info`` or
     $ cat ~/.ssh/authorized_keys
     command="borg serve --restrict-to-path /path/to/repo",no-pty,no-agent-forwarding,no-port-forwarding,no-X11-forwarding,no-user-rc ssh-rsa AAAAB3[...]
 
+    # Set a BORG_XXX environment variable on the "borg serve" side
+    $ cat ~/.ssh/authorized_keys
+    command="export BORG_XXX=value; borg serve [...]",restrict ssh-rsa [...]
 
 .. include:: usage/upgrade.rst.inc