ref-manual: Edits to the packageinfo class.

Forgot the part about the class being automatically enabled
when using the Hob.

(From yocto-docs rev: e15b52fc703f2bc4c02e2afe05de1cc4d057b685)

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

@@ -1336,6 +1336,12 @@
         gives a BitBake user interface the ability to retrieve information
         about output packages from the <filename>pkgdata</filename> files.
     </para>
+
+    <para>
+        This class is enabled automatically when using the
+        <ulink url='&YOCTO_HOME_URL;/tools-resources/projects/hob'>Hob</ulink>
+        user interface.
+    </para>
 </section>
 
 <section id='ref-classes-perl'>