docs..transaction_iso

2023
Tensor - Backend School - Transactions of 50:51
    https://www.youtube.com/watch?v=c9iF_4Xag4E
    ! 18:00 - Repeatable Read is not impl-ted in pg
    ! 24:00 - begin transaction <iso-level>;
    !         set transaction <iso-level>; -- docs..sql-set-transaction, change for current tr-n
    !         set session characteristics as transaction <iso-level>;