api: ++description

issue: https://github.com/thorvg/thorvg/issues/2494

inc/thorvg.h

@@ -631,6 +631,8 @@ public:
      * The Canvas rendering can be performed asynchronously. To make sure that rendering is finished,
      * the sync() must be called after the draw() regardless of threading.
      *
+     * @retval Result::InsufficientCondition: The canvas is either already in sync condition or in a damaged condition (a draw is required before syncing).
+     *
      * @see Canvas::draw()
      */
     virtual Result sync() noexcept;

src/bindings/capi/thorvg_capi.h

@@ -719,6 +719,7 @@ TVG_API Tvg_Result tvg_canvas_draw(Tvg_Canvas* canvas);
 *
 * \return Tvg_Result enumeration.
 * \retval TVG_RESULT_INVALID_ARGUMENT An invalid Tvg_Canvas pointer.
+* \retval TVG_RESULT_INSUFFICIENT_CONDITION @p canvas is either already in sync condition or in a damaged condition (a draw is required before syncing).
 *
 * \see tvg_canvas_draw()
 */

src/renderer/tvgCanvas.h

@@ -129,7 +129,7 @@ struct Canvas::Impl
             return Result::Success;
         }
 
-        return Result::InsufficientCondition;
+        return Result::Unknown;
     }
 
     Result viewport(int32_t x, int32_t y, int32_t w, int32_t h)