ref-manual: add more undocumented classes

(From yocto-docs rev: ac3cfb6ad67fc6963ebe50fa4451297772a1c2e7)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/ref-classes.xml

@@ -717,12 +717,16 @@
         deploy.bbclass
         distrodata.bbclass
         dummy.bbclass
+        fontcache.bbclass
         gconf.bbclass
         gettext.bbclass
         gnomebase.bbclass
         gnome.bbclass
+        grub-efi.bbclass
+        gsettings.bbclass
         gtk-doc.bbclass
         gtk-icon-cache.bbclass
+        gtk-immodules-cache.bbclass
         gzipnative.bbclass
         icecc.bbclass
         image-empty.bbclass
@@ -744,6 +748,7 @@
         logging.bbclass
         meta.bbclass
         metadata_scm.bbclass
+        migrate_localcount.bbclass
         mime.bbclass
         mirrors.bbclass
         multilib*.bbclass
@@ -755,12 +760,14 @@
         packageinfo.bbclass
         patch.bbclass
         perlnative.bbclass
+        pixbufcache.bbclass
         pkg_distribute.bbclass
         pkg_metainfo.bbclass
         populate_sdk*.bbclass
         prexport.bbclass
         primport.bbclass
         prserv.bbclass
+        ptest.bbclass
         python-dir.bbclass
         pythonnative.bbclass
         qemu.bbclass
@@ -777,6 +784,7 @@
         sstate.bbclass
         staging.bbclass
         syslinux.bbclass
+        systemd.bbclass
         terminal.bbclass
         tinderclient.bbclass
         toolchain-scripts.bbclass