ref-manual: Updated the rootfs*.bbclass description.

Fixes [YOCTO #8298]

Added "rootfsdebugfile" class to the list of classes in this
general class description.

(From yocto-docs rev: ad2f6e1d22c9453b0d28cdc0b53558efa1dcc212)

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

documentation/ref-manual/ref-classes.xml

@@ -3037,6 +3037,11 @@
                 The <filename>rootfs_ipk</filename> class, which supports
                 creation of root filesystems for images built using
                 <filename>.ipk</filename> packages.</para></listitem>
+            <listitem><para>
+                The <filename>rootfsdebugfiles</filename> class, which installs
+                additional files found on the build host directly into the
+                root filesystem.
+                </para></listitem>
         </itemizedlist>
     </para>