doc/go1.13: document removal of NaCl targets in Go 1.14

Go 1.14 will no longer run on Native Client (NaCl). Updating the 1.13
release notes to indicate this. See #30439.

Fixes #32948

Change-Id: Ia147bb053adc098bd696dbdb01553c23222f8134
Reviewed-on: https://go-review.googlesource.com/c/go/+/185537Reviewed-by: Andrew Bonventre <andybons@golang.org>

doc/go1.13.html

@@ -101,6 +101,10 @@ Do not send CLs removing the interior tags from such phrases.
 
 <h2 id="ports">Ports</h2>
 
+<p>
+  Go 1.13 is the last release that will run on Native Client (NaCl).
+</p>
+
 <h3 id="aix">AIX</h3>
 
 <p><!-- CL 164003, CL 169120 -->