units: do more reordering of ordering config

units: do more reordering of ordering config

No functional change, just a cleanup to make the subsequent changes easier to
see. This is a continuation of 9810e419425263bde86787bc21251f1ad3c35628

> The block is reordered and split to have:
>    1. description + documentation
>    2. (optionally) conditions
>    3. all the dependencies

The dependencies for shutdown.target are listed separately because they are the
other deps are for startup, and shutdown.target only matter much later.