net/http: document concurrency expectations for Request.Body

net/http: document concurrency expectations for Request.Body

This is primarily aimed at client requests where the user can supply
their own io.ReadCloser, but also clarifies server request behavior.
A server request body can be one of:

- *body
- *http2RequestBody
- *expectContinueReader
- *maxBytesReader

Of those, *expectContinueReader did not meet these expectations, so this
change also removes the data race.

Change-Id: Id4f1ae573d938347b1123a7b612b271aabb045a4
Reviewed-on: https://go-review.googlesource.com/c/go/+/251087
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
Trust: Damien Neil <dneil@google.com>
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Go Bot <gobot@golang.org>