journald: be a bit more verbose when vacuuming

Vacuuming behaviour is a bit confusing, and/or we have some bugs,
so those additional messages should help to find out what's going
on. Also, rotation of journal files shouldn't be happening too
often, so the level of the messages is bumped to info, so that
they'll be logged under normal operation.
man: add not to not use -x in bug reports