doc: add http.NoBody and Transport change to go1.8.html

Fixes #18257

Change-Id: Iafba5aa0a0641cfe36351bb0e4b57f20257fdc4e
Reviewed-on: https://go-review.googlesource.com/34210
Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.8.html

@@ -1320,6 +1320,22 @@ <h3 id="minor_library_changes">Minor changes to the library</h3>
         to use IPv4 as a backup if it looks like IPv6 might be
         failing.
       </li>
+
+      <li> <!-- CL 31726 -->
+        The <a href="/pkg/net/http/#Transport"><code>Transport</code></a>
+        no longer reads a byte of a non-nil
+        <a href="/pkg/net/http/#Request.Body"><code>Request.Body</code></a>
+        when the
+        <a href="/pkg/net/http/#Request.ContentLength"><code>Request.ContentLength</code></a>
+        is zero to determine whether the <code>ContentLength</code>
+        is actually zero or just undefined.
+        To explicitly signal that a body has zero length,
+        either set it to <code>nil</code>, or set it to the new value
+        <a href="/pkg/net/http/#NoBody"><code>NoBody</code></a>.
+        The new <code>NoBody</code> value is intended for use by <code>Request</code>
+        constructor functions; it is used by
+        <a href="/pkg/net/http/#NewRequest"><code>NewRequest</code></a>.
+      </li>
     </ul>
 
   </dd>