ipv6: don't use internal/iana's constants in documentation

Change-Id: I8f41ef0d3eefecb26d073e21bc1d5b7b6a51d259
Reviewed-on: https://go-review.googlesource.com/15053
Reviewed-by: Ian Lance Taylor <iant@golang.org>

ipv6/doc.go

@@ -42,7 +42,7 @@
 // The outgoing packets will be labeled DiffServ assured forwarding
 // class 1 low drop precedence, known as AF11 packets.
 //
-//			if err := ipv6.NewConn(c).SetTrafficClass(DiffServAF11); err != nil {
+//			if err := ipv6.NewConn(c).SetTrafficClass(0x28); err != nil {
 //				// error handling
 //			}
 //			if _, err := c.Write(data); err != nil {
@@ -124,7 +124,7 @@
 //
 // The application can also send both unicast and multicast packets.
 //
-//		p.SetTrafficClass(DiffServCS0)
+//		p.SetTrafficClass(0x0)
 //		p.SetHopLimit(16)
 //		if _, err := p.WriteTo(data[:n], nil, src); err != nil {
 //			// error handling