docs(api) added note for CAS about prevIndex=0 and `cause` meaning in server response

I have also noted that

curl -L http://127.0.0.1:4001/v2/keys/foo?prevIndex=8&prevValue=two

leads to weird cause `[two != one] [8 != 8]`. Not sure if it is intended or it is not a viable example.

Thank you!