Can server send PRIORITY_UPDATE?

[LPardue]

Migrating this issue originally reported by @dikhonov - kazuho/draft-kazuho-httpbis-priority#121 (comment)

Section 5 discusses how client can re-prioritize using the PRIORITY_UPDATE frame and Section 6 discusses how the server can override priority using the priority header. It seems, then, that the server cannot send the PRIORITY_UPDATE frame, but it is not forbidden in the draft.

My reply on the original issue was

Thanks for raising the issue. This is a gap in the spec and will need filling. The answer depends a little on the direction that the design takes in the WG; the salient question, I think, is: what is the collaboration story for reprioritization?

Once there is a answer, we can lock down some of the normative text in the TODO placeholder

TODO: add more description of how to handle things like receiving PRIORITY_UPDATE 
on wrong stream, a PRIORITY_UPDATE with an invalid ID, etc.wro