doc: mention testing.MainStart signature change

Fixes #18766

Change-Id: Ic0f72f3b7bbccd0546692993c4ed414f8c88c1c6
Reviewed-on: https://go-review.googlesource.com/35573Reviewed-by: Russ Cox <rsc@golang.org>

doc/go1.8.html

@@ -1645,6 +1645,17 @@ crypto/x509: return error for missing SerialNumber (CL 27238)
       and only the overall execution of the test binary would fail.
     </p>
 
+    <p><!-- CL 32455 -->
+      The signature of the
+      <a href="/pkg/testing/#MainStart"><code>MainStart</code></a>
+      function has changed, as allowed by the documentation. It is an
+      internal detail and not part of the Go 1 compatibility promise.
+      If you're not calling <code>MainStart</code> directly but see
+      errors, that likely means you set the
+      normally-empty <code>GOROOT</code> environment variable and it
+      doesn't match the version of your <code>go</code> command's binary.
+    </p>
+
   </dd>
 </dl>