math: add function examples.
math: add function examples.

The function Modf lacks corresponding examples.

Change-Id: Id93423500e87d35b0b6870882be1698b304797ae
Reviewed-on: https://go-review.googlesource.com/c/go/+/231097
Run-TryBot: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

golang/go,nawawi/go,cloudflare/go,nawawi/go,CAFxX/go,golang/go,golang/go,akutz/go,akutz/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,akutz/go,nawawi/go,golang/go,akutz/go,akutz/go,cloudflare/go,CAFxX/go,cloudflare/go,CAFxX/go,nawawi/go,nawawi/go,nawawi/go,nawawi/go,akutz/go,golang/go,cloudflare/go,cloudflare/go,cloudflare/go,golang/go,akutz/go,golang/go,cloudflare/go,akutz/go,CAFxX/go,CAFxX/go