Update engine_traits crate docs (#6121)
Update engine_traits crate docs (#6121)

Signed-off-by: Brian Anderson <2709ac8f6d3dc60d22e49a091caa2d2f10738e41@gmail.com>
BusyJay/tikv,BusyJay/tikv,pingcap/tikv,pingcap/tikv,BusyJay/tikv,c4pt0r/tikv,BusyJay/tikv