drm: add docs about the IN_FORMATS plane property

This is a standard property attached to planes in drm_universal_plane_init
when drm_mode_config.allow_fb_modifiers is true.
Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Daniel Stone <daniel@fooishbar.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/mHOtz80Wa-x3A5Lz5ETTM65VUMfW1j6b-iCuvIy1RzOM1EloHPqAb2MnbU8ygZi0ABaLrxuN6pzu0v3QRn83125C0bXuHoo-DQWWEd80fdw=@emersion.fr

drivers/gpu/drm/drm_blend.c

@@ -183,6 +183,12 @@
  *		 plane does not expose the "alpha" property, then this is
  *		 assumed to be 1.0
  *
+ * IN_FORMATS:
+ *	Blob property which contains the set of buffer format and modifier
+ *	pairs supported by this plane. The blob is a drm_format_modifier_blob
+ *	struct. Without this property the plane doesn't support buffers with
+ *	modifiers. Userspace cannot change this property.
+ *
  * Note that all the property extensions described here apply either to the
  * plane or the CRTC (e.g. for the background color, which currently is not
  * exposed and assumed to be black).