Fix checked operations documentation.

[ocstl]

Closes #13 . Remove the implication that wrapping is the default behaviour.

The documentation mentions "underflow" (for example, in CheckedSub), which I left as is. But Rust's documentation tends to refer to "overflow" exclusively (see i32::checked_sub).

I've recently been made aware that overflow is probably the correct term in both cases for integers (positive and negative), while underflow is strictly reserved for floating points. Either way, it's not a big deal and I'm not bothered by it, as the meaning is clear, but I thought I should mention it.

[cuviper]

Let's change "underflow" -- the reference calls it "overflow" for results less than the minimum:
https://doc.rust-lang.org/reference/expressions/operator-expr.html#overflow

... and "underflow" isn't found in the reference at all!

[ocstl]

I guess the reference settles that!

I also missed the CheckedEuclid methods in the first commit, but they are now fixed as well.

[cuviper]

Thanks!