Improve comment message of jihulab-bot

1. Clarify sub-labels in child labels reminder. Refer: gitlab-cn/gitlab#1154 (comment 590753)
2. Clarify type labels in type labels reminder. Refer: customerdot#33 (comment 590751)
3. Unified the greeting message. Refer: https://jihulab.com/gitlab-cn/internal/customers-jihulab-com/-/issues/195#note_597992

/cc @qianzhangxa