doc/go1.7.html: the 1.6.3 release supports Sierra

Updates #16354
Updates #16272

Change-Id: I73e8df40621a0a17a1990f3b10ea996f4fa738aa
Reviewed-on: https://go-review.googlesource.com/25014
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

doc/go1.7.html

@@ -76,8 +76,9 @@ This change has no effect on the correctness of existing programs.
 
 <p>
 Go 1.7 adds support for macOS 10.12 Sierra.
-Binaries built with previous versions of Go will not work correctly on
-Sierra.
+This support was backported to Go 1.6.3.
+Binaries built with versions of Go before 1.6.3 will not work
+correctly on Sierra.
 </p>
 
 <p>