doc/go1.18: document type parameter name restriction

For golang#47694.

Change-Id: I00862f987a0ff9f71e0295ce4320e6f9a6a4332f
Reviewed-on: https://go-review.googlesource.com/c/go/+/376414
Trust: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.18.html

@@ -108,6 +108,12 @@ <h3 id="generics">Generics</h3>
       interface type with a non-empty method set. Whether this will
       ever be permitted is unclear at present.
     </li>
+    <li><!-- https://golang.org/issue/50481 -->
+      A generic type or function currently may declare at most one blank (<code>_</code>)
+      type parameter name. Note that it is always possible to use an arbitrary new
+      (unused) identifier in place of a blank type parameter name.
+      We plan to remove this restriction in Go 1.19.
+    </li>
   </ul>
 </p>