tidb: warn limitations of placement rules dict syntax (#7686)
tidb: warn limitations of placement rules dict syntax (#7686)

* tidb: warn limitations of placement rules dict syntax

Signed-off-by: xhe <3cc054ec827480b50ed66e70bec75f34d9e09619@gmail.com>

* *: reword

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

* another rewording

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

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