man/sd_notify: change recommendations about unsupported notifications

man/sd_notify: change recommendations about unsupported notifications

In principle, arbitrary notifications may be sent via sd_notify. But in
practice, this is not useful at all, since the manager only accepts
notifications from services and ignores anything except a few specific
ones. The others will be logged if debugging is enabled. OTOH, the manager
produces EXIT_STATUS, but nothing in systemd looks at it, which is rather
confusing.

So remove the recommendation to use X_ prefixes, and instead say that other
messages will be ignored. Also, mention that mkosi uses this. Having an example
may be useful to understand what is going on.

Strangely, this is the first reference to mkosi in our man pages. Even more
strangely, debian is the only place which hosts the mkosi man page (among
the sites we have definitions for), so I linked to that version.