make_list -> get_part_of_board

renamed function to make it more descriptive of its actual function
check_diagonals_partially massive refactoring

there were previously still some parts of check_diagonals_partially
that kept it somewhat behaving like it's old form, get_diagonals.
This refactoring makes just do what it's supposed to do. It also
includes code cleanups, and a few comments
check_diagonals_partially documentation updates

numerous documenation updates to properly document this function
now that it has transition from get_diagonals ->
check_diagonals_partially.
I also added a few clarifications while I was at it