man: User=, Group= *never* work for mount units (#7602)

man: User=, Group= *never* work for mount units (#7602)

Old text:

> Note that the User= and
> Group= options are not particularly useful for mount units specifying a
> "Type=" option or using configuration not specified in /etc/fstab;
> mount(8) will refuse options that are not listed in /etc/fstab if it is
> not run as UID 0.

However I recently learnt the following:

> The mount program does not read the /etc/fstab file if both device
> and dir are specified.

Therefore, if both device and dir are specified, the `user` or `users`
options in `fstab` will not have any effect.  Run as a normal user,
you will always see

    mount: only root can do that

Fix the explanation in the man page.

Also make sure to markup User= and Group= with <varname>.