sound-naming-spec: Add cellbroadcast message

This are sufficiently distinct from instant messages and emails that it makes sense for them to be able to have a different sound.

I was on the fence whether this should go into "alerts" but those are meant things that "may have a major impact on the system or their current use" so message- looked like the better fit.

A user will be feedbackd but the event looks important enough to have a distinct name so other implementations can make use of the same sound from the sound theme.