Specify some kind of fallback-sizes-value-was-used state tracking

[eeeps]

Follow up from #27...

Right now we state that Sec-CH-Width must not be sent if a fallback size was used, and point at the step in the "parse a sizes attribute" algorithm that establishes whether a fallback size was used or not, but don't specify how that state is tracked across time between when the "parse a sizes attribute" algorithm is run and when "append client hints to request" is run.

I think... we probably need to do that? Open to hearing that we don't of course. In which case I'll remove the note in the spec.

[yoavweiss]

Yeah, makes sense to properly pass that state between the two algorithms. (and apologies for my slowness)