docs: add a note about selectively overriding default dependencies

docs: add a note about selectively overriding default dependencies

This documents that explicit `Before=`/`After=` dependencies can be
used to selectively override implicit ordering coming from default
dependencies. That allows for more granular control compared to the
already documented `DefaultDependencies=no` option.

The alternative approach came up in a discussion around the ordering
of `boot-complete.target`, so this also adds an explicit suggestion
in that direction to the "Automatic Boot Assessment" documentation.

Ref: https://lists.freedesktop.org/archives/systemd-devel/2022-September/048330.html