man: reword list-dependencies description

The previous version was a bit too vague. It is better
to simply list all dependency types that are followed.

Previous version also made an emphasis on dependencies introduced by
configuration.  But this command (or systemd) don't care about this
distinction between configured and automatically added dependencies at
all. This distinctionis removed from the main description, and an
explanatory paragraph is added to remind the user that all
dependencies are shown, no matter where they come from.