[release-branch.go1.7] doc/devel/release.html: document go1.6.3 doesn't actually support macOS Sierra

Updates #17824.

Change-Id: I73cf89c21b418158c7014c3271cd1103a17a5c86
Reviewed-on: https://go-review.googlesource.com/32882Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-on: https://go-review.googlesource.com/32885Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>