drm: Fix drm_rect documentation

The 'struct' keyword was missing so struct drm_rect documentation never
ended up in the generated docs.

Also move the drm_rect documentations to a new section alognside the
various helper functions and add a short description about the intended
purpose of drm_rect.

v2: Move to new section and add general description

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/include/drm/drm_rect.h b/include/drm/drm_rect.h
index 64fa265..d128629 100644
--- a/include/drm/drm_rect.h
+++ b/include/drm/drm_rect.h
@@ -25,7 +25,14 @@
 #define DRM_RECT_H
 
 /**
- * drm_rect - two dimensional rectangle
+ * DOC: rect utils
+ *
+ * Utility functions to help manage rectangular areas for
+ * clipping, scaling, etc. calculations.
+ */
+
+/**
+ * struct drm_rect - two dimensional rectangle
  * @x1: horizontal starting coordinate (inclusive)
  * @x2: horizontal ending coordinate (exclusive)
  * @y1: vertical starting coordinate (inclusive)