resolve: update comment, avoid alarming wrongness

resolve: update comment, avoid alarming wrongness

`systemd-resolved.service` runs as `User=systemd-resolved`, and uses certain
Capabilit{y,ies} magic. By my understanding, this means it is started with a
number of "privileges".  Indeed, `capabilities(7)` explains

> Linux divides  the  privileges  traditionally
> associated  with  superuser into distinct units, known as capabilities,
> which can be independently enabled and disabled."

This situation appears to contradict our current code comment which said

> If we are not running as root we assume all privileges are already dropped.

This appears to be a confusion in the comment only.  The rest of the code
tells a much clearer story.  (Don't ask me if the story is correct.
`capabilities(7)` scares me).  Let's tweak the comment to make it consistent
and avoid worrying readers about this.