Expand on motivation for the encoding

[martinthomson]

Explain why you probably shouldn't just use UTF-8.

[reschke]

...for new header fields?

[mnot]

Marking this as editorial, as I think it just means we need to point to 7230's restriction on header field values.

[reschke]

We already say:

"By default, header field values in HTTP messages ([RFC7230]) cannot directly carry characters outside the US-ASCII coded character set ([RFC0020])."

...which hints at the fact that RFC 7230 puts non-ASCII octets into the "obs-text" production. I'll try to expand a bit on this.

[reschke]

I believe this can be closed with the changes in 5ec7eed