Clarify SDL_ConvertEventToRenderCoordinates() documentation

Fixes https://github.com/libsdl-org/SDL/issues/14360

include/SDL3/SDL_render.h

@@ -1704,7 +1704,7 @@ extern SDL_DECLSPEC bool SDLCALL SDL_RenderCoordinatesToWindow(SDL_Renderer *ren
  *
  * \param renderer the rendering context.
  * \param event the event to modify.
- * \returns true on success or false on failure; call SDL_GetError() for more
+ * \returns true if the event is converted or doesn't need conversion, or false on failure; call SDL_GetError() for more
  *          information.
  *
  * \threadsafety This function should only be called on the main thread.