br: add collation restriction (#3646)
br: add collation restriction (#3646)

* br: add collation restriction

* add more details

* address comment

* Update br/backup-and-restore-tool.md

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

* Update br/backup-and-restore-tool.md

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

Co-authored-by: TomShawn <3d0966147538bc5ab020e8dffa2c0b746f9728a6@users.noreply.github.com>
Co-authored-by: ti-srebot <354121b1fcd840b60f1eb3b56f3ea5cc78ec06dd@users.noreply.github.com>
pingcap/docs-cn,pingcap/docs-cn