doc: document new math.Fma function
doc: document new math.Fma function

This accidentally got committed - please review the whole paragraph
as if it was new.

Change-Id: I98e1db4670634c6e792d26201ce0cd329a6928b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/202579
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

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