doc/go1.12: release notes for math/bits

Change-Id: I930942c7e057a36332ac06762f6aadf07574a7d5
Reviewed-on: https://go-review.googlesource.com/c/152977Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.12.html

@@ -406,7 +406,7 @@ for {
 <dl id="math/bits"><dt><a href="/pkg/math/bits/">math/bits</a></dt>
   <dd>
     <p><!-- CL 123157 -->
-      TODO: <a href="https://golang.org/cl/123157">https://golang.org/cl/123157</a>: add extended precision Add, Sub, Mul, Div
+    New extended precision operations <a href="/pkg/math/bits/#Add"><code>Add</code></a>, <a href="/pkg/math/bits/#Sub"><code>Sub</code></a>, <a href="/pkg/math/bits/#Mul"><code>Mul</code></a>, and <a href="/pkg/math/bits/#Div"><code>Div</code></a> are available in <code>int</code>, <code>int32</code>, and <code>int64</code> versions.
     </p>
 
 </dl><!-- math/bits -->