doc/go1.14: mention new method mime/multipart.(*Reader).NextRawPart

Updates golang#29090
Updates golang#36878

Change-Id: I63f0eb583285d5c12f591cb704097cdf6a67b64f
Reviewed-on: https://go-review.googlesource.com/c/go/+/217128
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.14.html

@@ -532,6 +532,18 @@ <h2 id="library">Core library</h2>
 
 </dl><!-- math/big -->
 
+<dl id="mime/multipart"><dt><a href="/pkg/mime/multipart/">mime/multipart</a></dt>
+  <dd>
+    <p>
+      The
+      new <a href="/pkg/mime/multipart/#Reader"><code>Reader</code></a>
+      method <a href="/pkg/mime/multipart/#Reader.NextRawPart"><code>NextRawPart</code></a>
+      supports fetching the next MIME part without transparently
+      decoding <code>quoted-printable</code> data.
+    </p>
+  </dd>
+</dl><!-- mime/multipart -->
+
 <dl id="net/http"><dt><a href="/pkg/net/http/">net/http</a></dt>
   <dd>
     <p><!-- CL 200760 -->