update the tidb configuration documents. (#4056)
update the tidb configuration documents. (#4056)

* update the tidb configuration documents.

* Apply suggestions from code review

* Update command-line-flags-for-tidb-configuration.md

* Update command-line-flags-for-tidb-configuration.md

* Apply suggestions from code review

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