Missing Modulo tests

[Milner39]

Here in docs/math.md, the docs claim a Modulo function exists. But when you look at numeric_test.go you can see the tests jump from div to mul skipping mod.

I also think the documentation for the mod function could be improved as it's not always obvious which number is operated on by the other, as we have to do func x y and not x func y. But this is just IMO.