man: improve wording of systemctl's --after/--before

Commit 4a77ca7 was an attempt at fixing the wording of --after and --before,
but the new wording was unclear.

Split the combined --after/--before section into a separate section for
each, explicitly state what each option does, and add information about
how these lists are generated.

Reported-by: Andrey Borzenkov <arvidjaar@gmail.com>
Reported-by: Lennart Poettering <lennart@poettering.net>