update TiFlash doc about expr pushdown when TiDB new collation is enabled (#3687)
update TiFlash doc about expr pushdown when TiDB new collation is enabled (#3687)

* update TiFlash doc about expr pushdown when TiDB new collation is enabled

* Update tiflash/use-tiflash.md

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

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