Document Modulo

[pckilgore]

In reason, mod worked as an operator assert(4 mod 3 === 1).

Had a devil of a time realizing that, while still in Pervasives, rescript modulo must be used as a function assert(mod(4, 3) === 1)

But more importantly, mod is a pretty common integer operation and newcomers are going to drive themselves batty figuring out how to do it since they might not know to go fishing in Pervasives.

[ryyppy]

It's currently documented here, but I agree, it should have some mention in the integer docs as well until we finally get infix operator support for mathematical operators.