Add math.isfinite, math.isinf, and math.isnan functions

[sammygrey]

Changes

Add details for math.isfinite, math.isinf, and math.isnan functions alphabetically.

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[aetter]

Hey @sammygrey, thanks a bunch for the proactivity here. I actually added these three functions upstream earlier today based on the same DevForum report. I expect they'll trickle down into the docs within a day or so. I'm going to merge for now, but my (very similar) content will override it when it comes through.