output: update docs to reflect reality

The docs were outdated and weren't matching what the DRM backend does
(the only implementor of wlr_output_export_dmabuf).

include/wlr/types/wlr_output.h

@@ -391,8 +391,7 @@ size_t wlr_output_get_gamma_size(struct wlr_output *output);
 void wlr_output_set_gamma(struct wlr_output *output, size_t size,
 	const uint16_t *r, const uint16_t *g, const uint16_t *b);
 /**
- * Exports the output's current back-buffer as a DMA-BUF (ie. the buffer that
+ * Exports the last committed buffer as a DMA-BUF.
- * will be displayed on next commit).
  *
  * The caller is responsible for cleaning up the DMA-BUF attributes.
  */