[release-branch.go1.13] doc/1.13: remove draft note and make various fixes

Updates #33954

Change-Id: Idfe71bf825adcd7cbf70cd139b3e779963394ff6
Reviewed-on: https://go-review.googlesource.com/c/go/+/192105


Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>
(cherry picked from commit dec16794)
Reviewed-on: https://go-review.googlesource.com/c/go/+/192743


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