doc/go1.16: mention new debug/elf constants

For #40700

Change-Id: If105d2f043539bb0893f577a984f14ee3e7ca753
Reviewed-on: https://go-review.googlesource.com/c/go/+/285212
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

Author: ianlancetaylor

doc/go1.16.html

@@ -616,6 +616,16 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
   </dd>
 </dl><!-- crypto/x509 -->
 
+<dl id="debug/elf"><dt><a href="/pkg/debug/elf/">debug/elf</a></dt>
+  <dd>
+    <p><!-- CL 255138 -->
+      More <a href="/pkg/debug/elf/#DT_NULL"><code>DT</code></a>
+      and <a href="/pkg/debug/elf/#PT_NULL"><code>PT</code></a>
+      constants have been added.
+    </p>
+  </dd>
+</dl><!-- debug/elf -->
+
 <dl id="encoding/asn1"><dt><a href="/pkg/encoding/asn1">encoding/asn1</a></dt>
   <dd>
     <p><!-- CL 255881 -->