net/http: document cloning of Server.TLSConfig

Updates #22018

Change-Id: I8a85324e9d53dd4d279ed05cdb93f50d55cf767b
Reviewed-on: https://go-review.googlesource.com/86415Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/http/server.go

@@ -2394,9 +2394,17 @@ 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
-	Handler   Handler     // handler to invoke, http.DefaultServeMux if nil
-	TLSConfig *tls.Config // optional TLS config, used by ServeTLS and ListenAndServeTLS
+	Addr    string  // TCP address to listen on, ":http" if empty
+	Handler Handler // handler to invoke, http.DefaultServeMux if nil
+
+	// TLSConfig optionally provides a TLS configuration for use
+	// by ServeTLS and ListenAndServeTLS. Note that this value is
+	// cloned by ServeTLS and ListenAndServeTLS, so it's not
+	// possible to modify the configuration with methods like
+	// tls.Config.SetSessionTicketKeys. To use
+	// SetSessionTicketKeys, use Server.Serve with a TLS Listener
+	// instead.
+	TLSConfig *tls.Config
 
 	// ReadTimeout is the maximum duration for reading the entire
 	// request, including the body.