client: add detailed comment  and revise cancel (#3210)
client: add detailed comment  and revise cancel (#3210)

* add comment

Signed-off-by: Song Gao <disxiaofei@163.com>

* Apply suggestions from code review

Signed-off-by: Song Gao <disxiaofei@163.com>

Co-authored-by: JmPotato <8b5de5447502ed6f17214c89c1d18aaadbb434e8@gmail.com>

* address the comment

Signed-off-by: Song Gao <disxiaofei@163.com>

Co-authored-by: JmPotato <8b5de5447502ed6f17214c89c1d18aaadbb434e8@gmail.com>
Co-authored-by: Ti Prow Robot <8d93ecb880184f72dbfca8f10b11aa46074d2fca@users.noreply.github.com>
pingcap/pd,pingcap/pd