Section 5.12 and 5.15 in conflict over `channelType`.

[MarkCallow]

I reported this to you already @fluppeteer but want to record it formally so we don't forget about it.

Alexey spotted a conflict between section 5.12 and 5.15. 5.12 shows channelType as the 4 bits containing the channel Id. 5.15 defines channelType as encompassing the whole 8 bits with 4 flag bits and the channel id in the low 4 bits. I think 5.15 is correct.