net/http: mention ALPN in http.Server.TLSNextProto documentation

Make clear negotiation can happen via NPN or ALPN, similar to
http.Transport.TLSNextProto and x/net/http2.NextProtoTLS.

Change-Id: Ied00b842bc04e11159d6d2107beda921cefbc6ca
Reviewed-on: https://go-review.googlesource.com/23108Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/server.go

@@ -2082,7 +2082,7 @@ type Server struct {
 	MaxHeaderBytes int
 
 	// TLSNextProto optionally specifies a function to take over
-	// ownership of the provided TLS connection when an NPN
+	// ownership of the provided TLS connection when an NPN/ALPN
 	// protocol upgrade has occurred. The map key is the protocol
 	// name negotiated. The Handler argument should be used to
 	// handle HTTP requests and will initialize the Request's TLS