Added a paragraph and examples explaining when an implementation
may use fused floating-point operations (such as FMA) and how to
prevent operation fusion.

For #17895.

Change-Id: I64c9559fc1097e597525caca420cfa7032d67014
Reviewed-on: https://go-review.googlesource.com/40391Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>