dbus-daemon(1): Describe how session and system services are found

dbus-daemon(1): Describe how session and system services are found

For Unix, this partially duplicates the D-Bus Specification, but
provides more detail about the intention of each search path element.
It also documents the non-standardized path elements searched by the
reference implementation.

For Windows, there are no standardized path elements in the D-Bus
Specification (and it isn't clear how useful it would be to standardize
them, since Windows software that uses D-Bus tends to be installed
as an integrated "stack" with a bundled copy of a suitable dbus-daemon),
so we just document what the reference implementation does.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=99825
Reviewed-by: Philip Withnall <withnall@endlessm.com>
[smcv: fix formatting nitpicks]
Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>