Add output transform helpers docs

include/wlr/types/wlr_output.h

@@ -118,8 +118,18 @@ bool wlr_output_cursor_move(struct wlr_output_cursor *cursor,
 	double x, double y);
 void wlr_output_cursor_destroy(struct wlr_output_cursor *cursor);
 
+
+/**
+ * Returns the transform that, when composed with `tr`, gives
+ * `WL_OUTPUT_TRANSFORM_NORMAL`.
+ */
 enum wl_output_transform wlr_output_transform_invert(
 	enum wl_output_transform tr);
+
+/**
+ * Returns a transform that, when applied, has the same effect as applying
+ * sequentially `tr_a` and `tr_b`.
+ */
 enum wl_output_transform wlr_output_transform_compose(
 	enum wl_output_transform tr_a, enum wl_output_transform tr_b);