add doc for broadcast join (#4636)
add doc for broadcast join (#4636)

* add doc for broadcast join

* refine wording

* Update tiflash/use-tiflash.md

* Update tiflash/use-tiflash.md

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