doc/go1.16: mention new vet check for asn1.Unmarshal

This vet check was added in CL 243397.

For #40700.

Change-Id: Ibff6df9395d37bb2b84a791443578009f23af4fb
GitHub-Last-Rev: e47c38f6309f31a6de48d4ffc82078d7ad45b171
GitHub-Pull-Request: golang/go#44147
Reviewed-on: https://go-review.googlesource.com/c/go/+/290330
Trust: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

doc/go1.16.html

@@ -378,6 +378,16 @@ <h4 id="vet-frame-pointer">New warning for frame pointer</h4>
   fixes.
 </p>
 
+<h4 id="vet-asn1-unmarshal">New warning for asn1.Unmarshal</h4>
+
+<p><!-- CL 243397 -->
+  The vet tool now warns about incorrectly passing a non-pointer or nil argument to
+  <a href="/pkg/encoding/asn1/#Unmarshal"><code>asn1.Unmarshal</code></a>.
+  This is like the existing checks for
+  <a href="/pkg/encoding/json/#Unmarshal"><code>encoding/json.Unmarshal</code></a>
+  and <a href="/pkg/encoding/xml/#Unmarshal"><code>encoding/xml.Unmarshal</code></a>.
+</p>
+
 <h2 id="runtime">Runtime</h2>
 
 <p>