docs(tvix/store/fuse): add more comment to list_root arg

Forgot to squash this into cl/9217.

Change-Id: Id5a87cbe729592f3b53e8c329b6890519f5da319
Reviewed-on: https://cl.tvl.fyi/c/depot/+/9272
Tested-by: BuildkiteCI
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/store/src/bin/tvix-store.rs

@@ -92,6 +92,8 @@ enum Commands {
         path_info_service_addr: String,
 
         /// Whether to list elements at the root of the mount point.
+        /// This is useful if your PathInfoService doesn't provide an
+        /// (exhaustive) listing.
         #[clap(long, short, action)]
         list_root: bool,
     },