dbus-launch: revise recommendations and put them in an EXAMPLES section

dbus-launch: revise recommendations and put them in an EXAMPLES section

The first thing we should talk about is how to get a D-Bus session in
your X session - that's the common case.

Secondarily, we can tell command-line addicts how to have a D-Bus session.
Do not recommend --exit-with-session here, since that polls (and reads
from) stdin, which is harmful to precisely those command-line users!
Until we have some better tool, the best we can do here is note that
the dbus-daemon is not automatically terminated.

Reviewed-by: Will Thompson <will.thompson@collabora.co.uk>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=39197