clarify SharedSecret usage by comment

It's not used aside from `server` but it was decided to keep it in place
for backwards compatibility in PR #1544.

backend/app/main.go

@@ -23,7 +23,8 @@ type Opts struct {
 	CleanupCmd cmd.CleanupCommand `command:"cleanup"`
 	RemapCmd   cmd.RemapCommand   `command:"remap"`
 
-	RemarkURL    string `long:"url" env:"REMARK_URL" required:"true" description:"url to remark"`
+	RemarkURL string `long:"url" env:"REMARK_URL" required:"true" description:"url to remark"`
+	// SharedSecret is only used in server command, but defined for all commands for historical reasons
 	SharedSecret string `long:"secret" env:"SECRET" required:"true" description:"the shared secret key used to sign JWT, should be a random, long, hard-to-guess string"`
 
 	Dbg bool `long:"dbg" env:"DEBUG" description:"debug mode"`