man: mention systemd-user-runtime-dir in user@.service too

man: mention systemd-user-runtime-dir in user@.service too

_riotingpacifist was complaining on reddit [1] that systemd-user-runtime-dir
is not documented anywhere. So let's add the binary name as page alias.

[1] https://www.reddit.com/r/linux/comments/h086fd/why_linuxs_systemd_is_still_divisive_after_all/ftllr66/

This page should be in section 8, like all .service descriptions.

Also extend the text a bit to make it clearer that systemd --user is the same
executable but running in a different mode (which might be certainly a bit
confusing to users.)