This CL updates the bundled copy of x/net/http2 to include
https://golang.org/cl/17930 and enables the previously-skipped tests
TestTrailersServerToClient_h2 and TestTrailersServerToClient_Flush_h2.

It also updates the docs on http.Response.Trailer to describe how to
use it. No change in rules. Just documenting the old unwritten rules.
(there were tests locking in the behavior, and misc docs and examples
scattered about, but not on http.Response.Trailer itself)

Updates #13557

Change-Id: I6261d439f6c0d17654a1a7928790e8ffed16df6c
Reviewed-on: https://go-review.googlesource.com/17931
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Blake Mizerany <blake.mizerany@gmail.com>