Spec: document what auto-starting is, and recommend it

Spec: document what auto-starting is, and recommend it

For something we recommend, that is important enough to have its own
header flag, it doesn't have very good documentation. Redo the text
to suggest that auto-starting is the normal thing and
StartServiceByName is the oddity. That's usually a good principle
to follow, since it dodges time-of-check/time-of-use issues, and the
method call that you presumably wanted to do needs to handle errors
anyway.

Signed-off-by: Simon McVittie <simon.mcvittie@collabora.co.uk>
Reviewed-by: Philip Withnall <philip.withnall@collabora.co.uk>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=98671