doc/go1.14: document io/ioutil.TempDir's predictable prefix+suffix

Documents io/ioutil.TempDir's new ability to create
predictable prefixes and suffixes, derived from the argument
'pattern', separated by the last '*' in it.

References: CL 198488

Updates golang#36878

Change-Id: I92c52fcc7d480ce74746e99e6e85a04efb87294f
Reviewed-on: https://go-review.googlesource.com/c/go/+/217780
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.14.html

@@ -569,6 +569,16 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
   </dd>
 </dl><!-- go/build -->
 
+<dl id="io/ioutil"><dt><a href="/pkg/io/ioutil/">io/ioutil</a></dt>
+  <dd>
+    <p><!-- CL 198488 -->
+      <a href="/pkg/io/ioutil/#TempDir"><code>TempDir</code></a> can now create directories
+      whose names have predictable prefixes and suffixes. As with TempFile, if the pattern
+      contains a '*', the random string replaces the last '*'.
+    </p>
+  </dd>
+</dl>
+
 <dl id="log"><dt><a href="/pkg/log/">log</a></dt>
   <dd>
     <p><!-- CL 186182 -->