net/http: document Request.Host value in HTTP/2

[FiloSottile]

HTTP/2 has :authority rather than Host, but I presume the Request.Host filed is filled in with that value for HTTP/2 requests. That should be documented.

/cc @bradfitz

[bradfitz]

Seems like maybe over-documenting internal protocol details, but this is already kinda over-documented with HTTP/1 details, so... sure. Will send.

[gopherbot]

Change https://golang.org/cl/198457 mentions this issue: net/http: document that Request.Host includes HTTP/2 :authority