promtail: "matches" directive missing from journal documentation

[lethargosapatheia]

Is your feature request related to a problem? Please describe.
Incomplete documentation in promtail related to journal.

Describe the solution you'd like
Add the matches directive in the journal section of promtail, i.e. https://grafana.com/docs/loki/latest/send-data/promtail/configuration/#journal

Also add information about how matches works, meaning, adding keys-values separated by spaces should work, e.g.:

_SYSTEMD_UNIT=lxc.service _SYSTEMD_UNIT=some_other.service

The matches directive is partially documented here:
https://grafana.com/docs/loki/latest/send-data/promtail/scraping/#journal-scraping-linux-only

Describe alternatives you've considered
Irrelevant

[JStickler]

Grafana Labs is no longer developing Promtail, all new features are being developed in Alloy. As a result, we're not going to devote documentation resources to adding to the Promtail docs. I'm going to leave this open in case anyone in the Community wants to take this on.