doc: update documentation for systemd + PrivateTmp users

The PrivateTmp feature of systemd breaks the usage of /tmp for the
shared Unix domain socket between nginx and unicorn, so discourage the
use of /tmp in that case.

While we're at it, use consistent paths for everything and use an
obviously intended-for-user-customization "/path/to" prefix instead
of "/tmp"

ML-Ref: CAKLVLx_t+9zWMhquMWDfStrxS7xrNoGmN0ZDsjSCUE=VxU+oyQ@mail.gmail.com
Reported-by: David Wilkins <dwilkins@conecuh.com>