improve docs for borg with-lock (#8024)

docs: add example for borg with-lock

src/borg/archiver/lock_cmds.py

@@ -73,7 +73,11 @@ class LocksMixIn:
 
         with_lock_epilog = process_epilog(
             """
-        This command runs a user-specified command while the repository lock is held.
+        This command runs a user-specified command while locking the repository. For example:
+
+        ::
+
+            $ BORG_REPO=/mnt/borgrepo borg with-lock rsync -av /mnt/borgrepo /somewhere/else/borgrepo
 
         It will first try to acquire the lock (make sure that no other operation is
         running in the repo), then execute the given command as a subprocess and wait