manual: Clarify that limits.conf doesn't apply to systemd services. (#40267)

Say how systemd services can be set instead.

Sources:

* https://bugzilla.redhat.com/show_bug.cgi?id=754285
* https://ro-che.info/articles/2017-03-26-increase-open-files-limit



Signed-off-by: Niklas Hambüchen <mail@nh2.me>