add system variable 'tidb_enable_clustered_index' for v5.0 (#4385)
add system variable 'tidb_enable_clustered_index' for v5.0 (#4385)

* add system variable 'tidb_enable_clustered_index' for v5.0

* Update system-variables.md

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

* address comment

* change back to '和’

* refine words

* add doc for checking if table use clustered index.

* address comment

* remove comma

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