Update docs on ResponseWriter and Handler around concurrency.

Also add a test.

The Handler docs were old and used "object" a lot. It was also too
ServeMux-centric.

Fixes #13050
Updates #13659 (new issue found in http2 while writing the test)

Change-Id: I25f53d5fa54f1c9d579d3d0f191bf3d94b1a251b
Reviewed-on: https://go-review.googlesource.com/17982Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>