doc/go1.18: document union element restriction

For #47694

Change-Id: I9af871a4a45b002e72629904011aac8f076617f1
Reviewed-on: https://go-review.googlesource.com/c/go/+/372974
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

doc/go1.18.html

@@ -103,6 +103,11 @@ <h3 id="generics">Generics</h3>
       embedding a type parameter in an interface type is not permitted.
       Whether these will ever be permitted is unclear at present.
     </li>
+    <li>
+      A union element with more than one term may not contain an
+      interface type with a non-empty method set. Whether this will
+      ever be permitted is unclear at present.
+    </li>
   </ul>
 </p>