net/http: clarify when it is safe to reuse a request
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.