journalctl: Make journalctl --user-unit= match on _SYSTEMD_USER_SLICE

journalctl:  Make journalctl --user-unit= match on _SYSTEMD_USER_SLICE

journalctl --unit= already did this, and allows you to tail all the logs
for a certain slice easily. It seemed only natural to make --user-unit
behave in a similar way.

The _SYSTEMD_USER_SLICE field was not documented as being added by
journald, so I have added that to the documentation too.

Furthermore, I have documented the existing behaviour of --unit= and the
new behaviour of --user-unit=

The behaviour was actually not documented before, so I am also OK with
removing the match for the --unit= command instead.  The user would then
have to manually provide _SYSTEMD_SLICE= filter to journalctl in both
cases. Both options work for me.