ref-manual, dev-manual: Edits to running automated tests section.

Applied a second round of review edits from Paul Eggleton
for the new "Performing Automated Runtime Testing" section.
I did some reorganization and some minor wording changes.

(From yocto-docs rev: fa8f8e5f0f6c1377a4fcafcd3d933af15ac01ff3)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/ref-classes.xml

@@ -943,8 +943,8 @@
         for images.
         The class handles loading the tests and starting the image.
         <note>
-            The current release of Yocto Project supports these tests
-            for QEMU images only.
+            Currently, there is only support for running these tests
+            under QEMU.
         </note>
     </para>