math: add Round and RoundToEven examples
math: add Round and RoundToEven examples

Change-Id: Ibef5f96ea588d17eac1c96ee3992e01943ba0fef
Reviewed-on: https://go-review.googlesource.com/131496
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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