drm: add comments for drm_encoder_funcs

Just basic verbiage.
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

include/drm/drm_crtc.h

@@ -437,6 +437,13 @@ struct drm_connector_funcs {
 	void (*force)(struct drm_connector *connector);
 };
 
+/**
+ * drm_encoder_funcs - encoder controls
+ * @reset: reset state (e.g. at init or resume time)
+ * @destroy: cleanup and free associated data
+ *
+ * Encoders sit between CRTCs and connectors.
+ */
 struct drm_encoder_funcs {
 	void (*reset)(struct drm_encoder *encoder);
 	void (*destroy)(struct drm_encoder *encoder);