improve system-variables documentation (#4598)
improve system-variables documentation (#4598)

* improve system-variables documentation

* fix lint issues

* Update system-variables.md

Co-authored-by: tangenta <fbddd308655086e575502d32d3450726b4280add@126.com>

* Update system-variables.md

Co-authored-by: tangenta <fbddd308655086e575502d32d3450726b4280add@126.com>

* Update system-variables.md

Co-authored-by: TomShawn <3d0966147538bc5ab020e8dffa2c0b746f9728a6@users.noreply.github.com>

* Update system-variables.md

Co-authored-by: TomShawn <3d0966147538bc5ab020e8dffa2c0b746f9728a6@users.noreply.github.com>

* Update system-variables.md

Co-authored-by: TomShawn <3d0966147538bc5ab020e8dffa2c0b746f9728a6@users.noreply.github.com>

* Update system-variables.md

Co-authored-by: TomShawn <3d0966147538bc5ab020e8dffa2c0b746f9728a6@users.noreply.github.com>

* Update system-variables.md

Co-authored-by: TomShawn <3d0966147538bc5ab020e8dffa2c0b746f9728a6@users.noreply.github.com>

Co-authored-by: tangenta <fbddd308655086e575502d32d3450726b4280add@126.com>
Co-authored-by: TomShawn <3d0966147538bc5ab020e8dffa2c0b746f9728a6@users.noreply.github.com>
pingcap/docs,pingcap/docs,pingcap/docs