docs: clarify `--update` and `--use-server-mod-time`
docs: clarify `--update` and `--use-server-mod-time`

It's likely a mistake to use `--use-server-modtime` if you're not also using `--update`. It might even make sense to emit a warning in the code when doing this, but for now, I made it more clear in the docs.

I also clarified how `--use-server-modtime` can be useful in the `--update` section.
ncw/rclone,ncw/rclone,ncw/rclone,ncw/rclone