description:
1.修改taskpool.excute接口说明
2.修改taskpool.cancel接口说明
3.新增taskpool.cancel各情况示例
4.新增优先级示例

issue:https://gitee.com/openharmony/docs/issues/I6ZOIBSigned-off-by: Nbuzhuyu <buzhuyu@huawei.com>