man: drop references to "syslog" and "syslog+console" from man page

These options are pretty much equivalent to "journal" and
"journal+console" anyway, let's simplify things, and drop them from the
documentation hence.

For compat reasons let's keep them in the code.

(Note that they are not 100% identical to 'journal', but I doubt the
distinction in behaviour is really relevant to keep this in the docs.
And we should probably should drop 'syslog' entirely from our codebase
eventually, but it's problematic as long as we semi-support udev on
non-systemd systems still.)