cmd/dockerd: add the link of "the documentation"

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
(cherry picked from commit 1a67e9572e95507791465e78e503ec21a4a32781)
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

cmd/dockerd/daemon.go

@@ -114,7 +114,7 @@ func (cli *DaemonCli) start(opts *daemonOptions) (err error) {
 
 	// return human-friendly error before creating files
 	if runtime.GOOS == "linux" && os.Geteuid() != 0 {
-		return fmt.Errorf("dockerd needs to be started with root. To see how to run dockerd in rootless mode with unprivileged user, see the documentation")
+		return fmt.Errorf("dockerd needs to be started with root privileges. To run dockerd in rootless mode as an unprivileged user, see https://docs.docker.com/go/rootless/")
 	}
 
 	system.InitLCOW(cli.Config.Experimental)