man: mention SYSTEMD_USER_WANTS

Also split into paragraphs for easier reading and make a few minor
corrections.
journal: pipe journalctl help output into a pager

journalctl help output might run off the screen, so be consistent
as other systemd tools do and pipe it into a pager.
Do not log all assert_return failures

% build/journalctl help
Assertion 'match_is_valid(data, size)' failed at ../src/journal/sd-journal.c:227, function sd_journal_add_match(). Ignoring.

Callers cannot be expect to check all arguments always.
nspawn: complain and continue if machine has same id

If --link-journal=host or --link-journal=guest is used, this totally
cannot work and we exit with an error. If however --link-journal=auto
or --link-journal=no is used, just display a warning.

Having the same machine id can happen if booting from the same
filesystem as the host. Since other things mostly function correctly,
let's allow that.

https://bugs.freedesktop.org/show_bug.cgi?id=68369