[docs] Complete hb-color docs a bit

src/hb-ot-color.h

@@ -66,6 +66,8 @@ hb_ot_color_palette_color_get_name_id (hb_face_t *face,
  * @HB_OT_COLOR_PALETTE_FLAG_USABLE_WITH_DARK_BACKGROUND: Flag indicating that the color
  *   palette is appropriate to use when displaying the font on a dark background such as black.
  *
+ * Flags that describe the properties of color palette.
+ *
  * Since: 2.1.0
  */
 typedef enum { /*< flags >*/
@@ -95,6 +97,8 @@ hb_ot_color_has_layers (hb_face_t *face);
 
 /**
  * hb_ot_color_layer_t:
+ * @glyph: the glyph ID of the layer
+ * @color_index: the palette color index of the layer
  *
  * Pairs of glyph and color index.
  *