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.

Fixes #19653

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

src/net/http/client.go

@@ -127,7 +127,10 @@ type RoundTripper interface {
 	// authentication, or cookies.
 	//
 	// RoundTrip should not modify the request, except for
-	// consuming and closing the Request's Body.
+	// consuming and closing the Request's Body. RoundTrip may
+	// read fields of the request in a separate goroutine. Callers
+	// should not mutate the request until the Response's Body has
+	// been closed.
 	//
 	// RoundTrip must always close the body, including on errors,
 	// but depending on the implementation may do so in a separate