man: try to explain different actions in tmpfiles a bit better

man: try to explain different actions in tmpfiles a bit better

- do not suggest that vendor configuration files should be in
  /etc, use /usr/lib/tmpfiles.d instead
- split the first example, because the text talked about "needing
  two directories", but then a smack attribute was also set, and
  on a different path, which looked like a typo. Replace that
  with the example from original patch [1] which added 't'.
- fix the example for /var/tmp/abrt. The 'x' line was redundant,
  because /var/tmp/abrt/* is already filtered because "d /var/tmp/abrt"
  overrides "d /var/tmp".

[1] http://permalink.gmane.org/gmane.comp.sysutils.systemd.devel/25051