net/http: mention RegisterOnShutdown in Server.Shutdown docs

Closes #21637

Change-Id: Icc3528572ea2a25e62757cc8fbbb9c3fa96a78b2
Reviewed-on: https://go-review.googlesource.com/59350Reviewed-by: Tom Bergan <tombergan@google.com>

src/net/http/server.go

@@ -2487,7 +2487,8 @@ var shutdownPollInterval = 500 * time.Millisecond
 // Shutdown does not attempt to close nor wait for hijacked
 // connections such as WebSockets. The caller of Shutdown should
 // separately notify such long-lived connections of shutdown and wait
-// for them to close, if desired.
+// for them to close, if desired. See RegisterOnShutdown for a way to
+// register shutdown notification functions.
 func (srv *Server) Shutdown(ctx context.Context) error {
 	atomic.AddInt32(&srv.inShutdown, 1)
 	defer atomic.AddInt32(&srv.inShutdown, -1)