docs(cfg): add usage tag for CookiePrefix config field

- Add usage description "cookie prefix. default: vb_" to JWTConfig.CookiePrefix

cfg/cfg.go

@@ -40,7 +40,7 @@ type JWTConfig struct {
 	RefreshExpiry time.Duration `json:"refresh_expiry" usage:"Refresh Token 有效期"`
 	Issuer        string        `json:"issuer" usage:"JWT 签发者"`
 	CookiePath    string        `json:"cookie_path" usage:"Cookie Path，默认 / 全站，可限定为 /api/ 等"`
-	CookiePrefix  string        `json:"cookie_prefix"`
+	CookiePrefix  string        `json:"cookie_prefix" usage:"cookie prefix. default: vb_"`
 }
 
 // InitAdminConfig 初始管理员配置