[DOC] add documents about full polling thread

[loongs-zhang]

After repeatedly watching https://www.bilibili.com/video/BV1VN4y1V7jV/?spm_id_from=333.788 and https://www.bilibili.com/video/BV1PG411b7eE/?spm_id_from=333.788, I still can't understand the thread model of full polling, and no relevant documents have been found on the brpc official website. Is there any master can add it ?

[Huixxi]

After repeatedly watching https://www.bilibili.com/video/BV1VN4y1V7jV/?spm_id_from=333.788 and https://www.bilibili.com/video/BV1PG411b7eE/?spm_id_from=333.788, I still can't understand the thread model of full polling, and no relevant documents have been found on the brpc official website. Is there any master can add it ?

I'll also try to learn it and may summarize a document.