doc: update developer notes wrt unix epoch time
doc: update developer notes wrt unix epoch time

Former-commit-id: 52400df2edea1bcdf6fed5624668e277125a6bb3
syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin2,syscoin/syscoin2,syscoin/syscoin,syscoin/syscoin