cmd/tailscale: add warning to help text of `--force-reauth` (#14778)

The warning text is adapted from https://tailscale.com/kb/1028/key-expiry#renewing-keys-for-an-expired-device .

There is already https://github.com/tailscale/tailscale/pull/7575 which presents a warning when connected over Tailscale, however the detection is done by checking SSH environment variables, which are absent within systemd's run0*. That means `--force-reauth` will happily bring down Tailscale connection, leaving the user in despair.

Changing only the help text is by no means a complete solution, but hopefully it will stop users from blindly trying it out, and motivate them to search for a proper solution.

*: https://www.freedesktop.org/software/systemd/man/devel/run0.html

Updates #3849

Signed-off-by: yejingchen <ye.jingchen@gmail.com>

cmd/tailscale/cli/up.go

@@ -139,7 +139,7 @@ func newUpFlagSet(goos string, upArgs *upArgsT, cmd string) *flag.FlagSet {
 		// Some flags are only for "up", not "login".
 		upf.BoolVar(&upArgs.json, "json", false, "output in JSON format (WARNING: format subject to change)")
 		upf.BoolVar(&upArgs.reset, "reset", false, "reset unspecified settings to their default values")
-		upf.BoolVar(&upArgs.forceReauth, "force-reauth", false, "force reauthentication")
+		upf.BoolVar(&upArgs.forceReauth, "force-reauth", false, "force reauthentication (WARNING: this will bring down the Tailscale connection and thus should not be done remotely over SSH or RDP)")
 		registerAcceptRiskFlag(upf, &upArgs.acceptedRisks)
 	}