documentation/poky-ref-manual/ref-classes.xml: Add packagegroup.bbclass

Add a short section about packagegroup.bbclass.

In addition to Paul's patch, I changed the title of the cross-reference
to be the actual title used in the YP Development Manual.

(From yocto-docs rev: bfd4db7e0ef61a2dd7159cdc841dcffc2d9c53d4)

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

documentation/poky-ref-manual/ref-classes.xml

@@ -262,6 +262,28 @@
     </para>
 </section>
 
+<section id='ref-classes-packagegroup'>
+    <title>Package Groups - <filename>packagegroup.bbclass</filename></title>
+
+    <para>
+        This class sets default values appropriate for package group recipes (such as 
+        <filename><link linkend='var-PACKAGES'>PACKAGES</link></filename>, 
+        <filename><link linkend='var-PACKAGE_ARCH'>PACKAGE_ARCH</link></filename>, 
+        <filename><link linkend='var-ALLOW_EMPTY'>ALLOW_EMPTY</link></filename>, 
+        and so forth. 
+        It is highly recommended that all package group recipes inherit this class.
+    </para>
+    <para>
+        For information on how to use this class, see the 
+        "<ulink url='&YOCTO_DOCS_DEV_URL;#usingpoky-extend-customimage-customtasks'>Customizing Images Using Custom Package Tasks</ulink>" 
+        section in the Yocto Project Development Manual.
+    </para>
+    <para>
+        Previously, this class was named <filename>task.bbclass</filename>.
+    </para>
+</section>
+
+
 <section id='ref-classes-package'>
     <title>Packaging - <filename>package*.bbclass</filename></title>
 
@@ -659,7 +681,6 @@
         sstate.bbclass
         staging.bbclass
         syslinux.bbclass
-        task.bbclass
         terminal.bbclass
         tinderclient.bbclass
         toolchain-scripts.bbclass