doc: mention generics in release notes

Also mention local types restriction.

We probably want to say more at some point, this is just a
placeholder to start.

Update #47631

Change-Id: I828e451e1e8504d21cb55c7132e9cb330b160a54
Reviewed-on: https://go-review.googlesource.com/c/go/+/364134
Trust: Keith Randall <khr@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

Author: randall77

doc/go1.18.html

@@ -29,6 +29,24 @@ <h2 id="language">Changes to the language</h2>
   TODO: complete this section
 </p>
 
+<h3 id="generics">Generics</h3>
+
+<p>
+Go 1.18 includes an implementation of generics as described
+by <a href="https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md">the
+proposal</a>.
+</p>
+
+<p>
+  The current generics implementation has the following limitations:
+  <ul>
+    <li><!-- issue 47631 -->
+      The Go compiler cannot currently handle type declarations inside generic functions
+      or methods. We hope to provide support for this feature in Go 1.19.
+    </li>
+  </ul>
+</p>
+
 <h2 id="ports">Ports</h2>
 
 <h3 id="freebsd">FreeBSD</h3>