[release-branch.go1.13] doc/go1.13: add id tag to note about removal of NaCl port in Go 1.14

This was in response to a post-merge review comment in
golang.org/cl/185537

Change-Id: I866b3882c8e83bf1fef60115cff5d1c6a9863f09
Reviewed-on: https://go-review.googlesource.com/c/go/+/186319


Reviewed-by: Andrew Bonventre <andybons@golang.org>
(cherry picked from commit b7e9c7a3)
Reviewed-on: https://go-review.googlesource.com/c/go/+/196377


Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Alexander Rakoczy <alex@golang.org>
Reviewed-by: Toshihiro Shiino <shiino.toshihiro@gmail.com>