refine explanation for general log (#4843)
refine explanation for general log (#4843)

* refine doc for general log

* address comment

* fix broken link

* Update system-variables.md

* Update system-variables.md

Co-authored-by: TomShawn <3d0966147538bc5ab020e8dffa2c0b746f9728a6@users.noreply.github.com>
pingcap/docs-cn,pingcap/docs-cn