drm/doc: move composition function docs to new section

Move drm_blend.c function reference from the KMS properties section to
the plane abstraction section. This makes the KMS properties section
more readable for user-space developers.
Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Vetter <daniel@ffwll.ch>
Cc: Pekka Paalanen <ppaalanen@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201216202222.48146-3-contact@emersion.fr

Documentation/gpu/drm-kms.rst

@@ -370,6 +370,12 @@ Plane Functions Reference
 .. kernel-doc:: drivers/gpu/drm/drm_plane.c
    :export:
 
+Plane Composition Functions Reference
+-------------------------------------
+
+.. kernel-doc:: drivers/gpu/drm/drm_blend.c
+   :export:
+
 Display Modes Function Reference
 ================================
 
@@ -472,9 +478,6 @@ Plane Composition Properties
 .. kernel-doc:: drivers/gpu/drm/drm_blend.c
    :doc: overview
 
-.. kernel-doc:: drivers/gpu/drm/drm_blend.c
-   :export:
-
 Damage Tracking Properties
 --------------------------