doc/go1.14: announce upcoming removal of darwin/386 port

Fixes #34749

Change-Id: Id97afc189ea387fc0fdd044140e30096594e185a
Reviewed-on: https://go-review.googlesource.com/c/go/+/202018Reviewed-by: Bryan C. Mills <bcmills@google.com>

doc/go1.14.html

@@ -50,6 +50,13 @@ TODO
   maintain it.
 </p>
 
+<h3 id="darwin">Darwin</h3>
+
+<p><!-- golang.org/issue/34749 -->
+  Go 1.14 is the last Go release to support 32-bit binaries on
+  macOS (the <code>darwin/386</code> port). They are no longer
+  supported by macOS, starting with macOS 10.15 (Catalina).
+</p>
 
 <h3 id="nacl">Native Client (NaCl)</h3>