cmd/tailscale/cli: flesh out "tailscale ssh" CLI docs
Per user feedback.
Fixes
Name: "ssh",
ShortUsage: "ssh [user@]<host> [args...]",
ShortHelp: "SSH to a Tailscale machine",
+ LongHelp: strings.TrimSpace(`
+
+The 'tailscale ssh' command is an optional wrapper around the system 'ssh'
+command that's useful in some cases. Tailscale SSH does not require its use;
+most users running the Tailscale SSH server will prefer to just use the normal
+'ssh' command or their normal SSH client.
+
+The 'tailscale ssh' wrapper adds a few things:
+
+* It resolves the destination server name in its arugments using MagicDNS,
+ even if --accept-dns=false.
+* It works in userspace-networking mode, by supplying a ProxyCommand to the
+ system 'ssh' command that connects via a pipe through tailscaled.
+* It automatically checks the destination server's SSH host key against the
+ node's SSH host key as advertised via the Tailscale coordination server.
+`),
Exec: runSSH,
}
