I wanted to clean the journal to remove about 400MB of old logs. Unfortunately the docs not showed me any examples. There was just the man journalctl mentioned.
A quick search brought up this:
Might be, if someone who likes to update quick docs, could use this as an inspiration 
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This two examples showed how simple it can be:
journalctl --disk-usage Archived and active journals take up 384.0M on disk.journalctl --vacuum-size=200M Deleted archived journal /run/log/Wow!
Thanks for that @ilikelinux .
It seems I had 2.3G of journalctl logs and the vacuum removed 2.1G of that.
journalctl --vacuum-time=1days
In addition to --vacuum-size=, --vacuum-time= removes archived journal files older than the specified timespan. Accepts the usual s (default), m, h, days, weeks, months, and years suffixes.
Doing some journalctl cleanup
sudo journalctl --rotate --vacuum-time=1days --vacuum-size=20M