doc: add Go 1.12 release note for trigonometric reductions in math

Worth mentioning because the results are not bit-for-bit identical.
This causes a test failure in github.com/fogleman/gg.

Updates #6794

Change-Id: I701f34927731fb5c658a1be271c04388e5e7e3f7
Reviewed-on: https://go-review.googlesource.com/c/157417


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>