The godoc for RoundTrip already specifies when it's ok to reuse a
request that contains a body: the caller must wait until RoundTrip
calls Close on Request.Body.

This CL adds a small clarification: If the request does not have a
body, it can be reused as long as the caller does not mutate the
Request until RoundTrip fails or the Response.Body is closed.

Fixes #19653

Change-Id: I56652a9369978d11650e2e6314104831c2ce5e78
Reviewed-on: https://go-review.googlesource.com/75671Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>