doc/go1.13: binary-only package are no longer supported

Change-Id: Ieb4a78730a0685627b4730381b72cbd01031c8fa
Reviewed-on: https://go-review.googlesource.com/c/go/+/183633
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>

doc/go1.13.html

@@ -366,6 +366,13 @@ go env -w GOSUMDB=off
   files may be searched for directives but ignored during build.
 </p>
 
+<p id="binary-only"><!-- CL 165746 -->
+  As <a href="/doc/go1.12#binary-only">announced</a> in the Go 1.12 release
+  notes, binary-only packages are no longer supported. Building a binary-only
+  package (marked with a <code>//go:binary-only-package</code> comment) now
+  results in an error.
+</p>
+
 <h3 id="compiler">Compiler toolchain</h3>
 
 <p><!-- CL 170448 -->