doc/go1.14: mention math/big.GCD argument change

Change-Id: Ib3b5f64471e7b9794b15a97ba86ba001f2c7d2ae
Reviewed-on: https://go-review.googlesource.com/c/go/+/216200
Reviewed-by: Robert Griesemer <[email protected]>

Author: bmkessler

doc/go1.14.html

@@ -417,6 +417,16 @@ <h2 id="library">Core library</h2>
 
 </dl><!-- math -->
 
+<dl id="math/big"><dt><a href="/pkg/math/big">math/big</a></dt>
+  <dd>
+    <p><!-- CL 164972 -->
+      The <a href="/pkg/math/big#GCD"><code>GCD</code></a> function
+      now allows the inputs <code>a</code> and <code>b</code> to be
+      zero or negative.
+    </p>
+
+</dl><!-- math/big -->
+
 <dl id="plugin"><dt><a href="/pkg/plugin/">plugin</a></dt>
   <dd>
     <p><!-- CL 191617 -->