http2: use RFC 723x as normative reference in docs

Replace references to the obsoleted RFC 2616 with references to RFC
7230 through 7235, to avoid unnecessary confusion.

Updates golang/go#21974

Change-Id: Idbe3e73199f0bef9dbdbe1b75c39382799cd646c
Reviewed-on: https://go-review.googlesource.com/94555
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

http2/http2.go

@@ -312,7 +312,7 @@ func mustUint31(v int32) uint32 {
 }
 
 // bodyAllowedForStatus reports whether a given response status code
-// permits a body. See RFC 2616, section 4.4.
+// permits a body. See RFC 7230, section 3.3.
 func bodyAllowedForStatus(status int) bool {
 	switch {
 	case status >= 100 && status <= 199:

http2/server.go

@@ -2285,7 +2285,7 @@ func (rws *responseWriterState) hasTrailers() bool { return len(rws.trailers) !=
 func (rws *responseWriterState) declareTrailer(k string) {
 	k = http.CanonicalHeaderKey(k)
 	if !ValidTrailerHeader(k) {
-		// Forbidden by RFC 2616 14.40.
+		// Forbidden by RFC 7230, section 4.1.2.
 		rws.conn.logf("ignoring invalid trailer %q", k)
 		return
 	}
@@ -2406,7 +2406,7 @@ const TrailerPrefix = "Trailer:"
 // after the header has already been flushed. Because the Go
 // ResponseWriter interface has no way to set Trailers (only the
 // Header), and because we didn't want to expand the ResponseWriter
-// interface, and because nobody used trailers, and because RFC 2616
+// interface, and because nobody used trailers, and because RFC 7230
 // says you SHOULD (but not must) predeclare any trailers in the
 // header, the official ResponseWriter rules said trailers in Go must
 // be predeclared, and then we reuse the same ResponseWriter.Header()
@@ -2790,7 +2790,7 @@ func (sc *serverConn) startPush(msg *startPushRequest) {
 }
 
 // foreachHeaderElement splits v according to the "#rule" construction
-// in RFC 2616 section 2.1 and calls fn for each non-empty element.
+// in RFC 7230 section 7 and calls fn for each non-empty element.
 func foreachHeaderElement(v string, fn func(string)) {
 	v = textproto.TrimString(v)
 	if v == "" {

http2/server_test.go

@@ -2877,9 +2877,9 @@ func testServerWritesTrailers(t *testing.T, withFlush bool) {
 		w.Header().Set("Trailer:post-header-trailer2", "hi2")
 		w.Header().Set("Trailer:Range", "invalid")
 		w.Header().Set("Trailer:Foo\x01Bogus", "invalid")
-		w.Header().Set("Transfer-Encoding", "should not be included; Forbidden by RFC 2616 14.40")
-		w.Header().Set("Content-Length", "should not be included; Forbidden by RFC 2616 14.40")
-		w.Header().Set("Trailer", "should not be included; Forbidden by RFC 2616 14.40")
+		w.Header().Set("Transfer-Encoding", "should not be included; Forbidden by RFC 7230 4.1.2")
+		w.Header().Set("Content-Length", "should not be included; Forbidden by RFC 7230 4.1.2")
+		w.Header().Set("Trailer", "should not be included; Forbidden by RFC 7230 4.1.2")
 		return nil
 	}, func(st *serverTester) {
 		getSlash(st)