Fix docs, remove outdated comments, remove overflowing divs/rems.

[NikVolf]

(overflowing_div/overflowing_rem is pointless for unsigned integers)

[sorpaas]

Not yet sure whether this is a good idea or not, but I just want to note that Rust's primitive type like u64 do have overflowing_div and overflowing_rem. Would we want to consider to add them still as helper functions, just so that, quoting Rust's docs, "all operations are accounted for in the overflowing operations"?