drm/docs: Include hdmi infoframe helper reference

Thierry created such nice kerneldocs, it's a shame we've left them
lingering!

For the fun of it also add a bit of kerneldoc to the header so that we
can also include that. Just in case someone adds kerneldoc in there.

Cc: Thierry Reding <[e-mail]>
Reviewed-by: Alex Deucher <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>

Documentation/DocBook/drm.tmpl

···
 !Iinclude/drm/drm_flip_work.h
 !Edrivers/gpu/drm/drm_flip_work.c
     </sect2>
+    <sect2>
+      <title>HDMI Infoframes Helper Reference</title>
+      <para>
+	Strictly speaking this is not a DRM helper library but generally useable
+	by any driver interfacing with HDMI outputs like v4l or alsa drivers.
+	But it nicely fits into the overall topic of mode setting helper
+	libraries and hence is also included here.
+      </para>
+!Iinclude/linux/hdmi.h
+!Edrivers/video/hdmi.c
+    </sect2>
   </sect1>
 
   <!-- Internals: kms properties -->

include/linux/hdmi.h

···
 	struct hdmi_vendor_infoframe hdmi;
 };
 
+/**
+ * union hdmi_infoframe - overall union of all abstract infoframe representations
+ * @any: generic infoframe
+ * @avi: avi infoframe
+ * @spd: spd infoframe
+ * @vendor: union of all vendor infoframes
+ * @audio: audio infoframe
+ *
+ * This is used by the generic pack function. This works since all infoframes
+ * have the same header which also indicates which type of infoframe should be
+ * packed.
+ */
 union hdmi_infoframe {
 	struct hdmi_any_infoframe any;
 	struct hdmi_avi_infoframe avi;