net/http: document ResponseWriter read-vs-write concurrency rules

net/http: document ResponseWriter read-vs-write concurrency rules

Summary: Go's HTTP/1.x server closes the request body once writes are
flushed. Go's HTTP/2 server supports concurrent read & write.

Added a TODO to make the HTTP/1.x server also support concurrent
read+write. But for now, document it.

Updates #15527

Change-Id: I81f7354923d37bfc1632629679c75c06a62bb584
Reviewed-on: https://go-review.googlesource.com/23011
Reviewed-by: Andrew Gerrand <adg@golang.org>