doc: explain how to use "go vet -shadow"

Fixes #29260

Change-Id: I419b74d06380113f4bd32b9aeb053c3be36208d5
Reviewed-on: https://go-review.googlesource.com/c/154584
Reviewed-by: Alan Donovan <[email protected]>

Author: ianlancetaylor

doc/go1.12.html

@@ -93,6 +93,16 @@ <h3 id="vet"><code>go tool vet</code> no longer supported</h3>
   vet</code> should work with all supported versions of Go.
 </p>
 
+<p>
+  As part of this change, the experimental <code>-shadow</code> option
+  is no longer available with <code>go vet</code>. Checking for
+  variable shadowing may now be done using
+  <pre>
+    go install golang.org/x/tools/go/analysis/passes/shadow/cmd/shadow
+    go vet -vettool=$(which shadow)
+  </pre>
+</p>
+
 <h3 id="gocache">Build cache requirement</h3>
 
 <p>