[release-branch.go1.14] doc/go1.14: mention Windows change for Open permissions

For #35033
For #36878

Change-Id: Ie15353322d5cfe7320199103ad9543fb89a842ed
Reviewed-on: https://go-review.googlesource.com/c/go/+/223957


Reviewed-by: Brendan Jackman <jackmanb@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
(cherry picked from commit e39de051)
Reviewed-on: https://go-review.googlesource.com/c/go/+/223962


Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>