net/http: document ErrServerClosed

Fixes #19085

Change-Id: Ib11b9a22ea8092aca9e1c9c36b1fb015dd555c4b
Reviewed-on: https://go-review.googlesource.com/36943Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/server.go

@@ -2613,6 +2613,8 @@ func (srv *Server) shouldConfigureHTTP2ForServe() bool {
 	return strSliceContains(srv.TLSConfig.NextProtos, http2NextProtoTLS)
 }
 
+// ErrServerClosed is returned by the Server's Serve, ListenAndServe,
+// and ListenAndServeTLS methods after a call to Shutdown or Close.
 var ErrServerClosed = errors.New("http: Server closed")
 
 // Serve accepts incoming connections on the Listener l, creating a