[Blog] Explain how bookie lastAddConfirm works

[hangc0276]

Motivation

Currently, Bookie's lastAddConfirm logic is a little complicated, and in some cases, the lastAddConfirm result is not a fixed value. When some users find this not a fixed value issue, they may be confused about the bookie's protocol, we need a blog deep into the detail of how bookie lastAddConfirm works and why lastAddConfirm is not a fixed value is reasonable.

YanZhao will take over this blog.