man: document unit load paths

man: document unit load paths

So far we didn't document control, transient, dbus config, or generator paths.
But those paths are visible to users, and they need to understand why systemd
loads units from those paths, and how the precedence hierarchy looks.
The whole thing is a bit messy, since the list of paths is quite long.
I made the tables a bit shorter by combining rows for the alternatives
where $XDG_* is set and the fallback.

In various places, tags are split like <element
  param="blah">
this. This is necessary to keep everyting in one logical XML line so that
docbook renders the table properly.

Replaces #8050.