ref-manual: Updated testimage and testsdk class descriptions.

I added notes indicating that the best practice for automated testing
is to inherit these classes by using the IMAGE_CLASSES variable instead
of the INHERIT variable.

(From yocto-docs rev: 70e18e93e96f283ae183f5f4468859bfca37c581)

Signed-off-by: Scott Rifenbark <srifenbark@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/ref-classes.xml

@@ -3502,6 +3502,14 @@ This check was removed for YP 2.3 release
         The classes handle loading the tests and starting the image.
         To use the classes, you need to perform steps to set up the
         environment.
+        <note><title>Tip</title>
+            Best practices include using
+            <link linkend='var-IMAGE_CLASSES'><filename>IMAGE_CLASSES</filename></link>
+            rather than
+            <link linkend='var-INHERIT'><filename>INHERIT</filename></link> to
+            inherit the <filename>testimage</filename> class for automated
+            image testing.
+        </note>
     </para>
 
     <para>
@@ -3541,6 +3549,14 @@ This check was removed for YP 2.3 release
         <literallayout class='monospaced'>
      $ bitbake -c testsdk image
         </literallayout>
+        <note><title>Tip</title>
+            Best practices include using
+            <link linkend='var-IMAGE_CLASSES'><filename>IMAGE_CLASSES</filename></link>
+            rather than
+            <link linkend='var-INHERIT'><filename>INHERIT</filename></link> to
+            inherit the <filename>testsdk</filename> class for automated
+            SDK testing.
+        </note>
     </para>
 </section>