net/http: enhance documentation for Server.Addr

Fixes golang/go#31249

Change-Id: I3280f8ab170ed31d4efb71106533e016d430d44c
Reviewed-on: https://go-review.googlesource.com/c/go/+/191557
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/server.go

@@ -2488,7 +2488,12 @@ func ServeTLS(l net.Listener, handler Handler, certFile, keyFile string) error {
 // A Server defines parameters for running an HTTP server.
 // The zero value for Server is a valid configuration.
 type Server struct {
-	Addr    string  // TCP address to listen on, ":http" if empty
+	// Addr optionally specifies the TCP address for the server to listen on,
+	// in the form "host:port". If empty, ":http" (port 80) is used.
+	// The service names are defined in RFC 6335 and assigned by IANA.
+	// See net.Dial for details of the address format.
+	Addr string
+
 	Handler Handler // handler to invoke, http.DefaultServeMux if nil
 
 	// TLSConfig optionally provides a TLS configuration for use