Updated SDL_StretchSurface() documentation

Fixes https://github.com/libsdl-org/SDL/issues/13135

(cherry picked from commit 7db0ac7380196f92af02503e64042710ec4f0204)

include/SDL3/SDL_surface.h

@@ -1279,10 +1279,11 @@ extern SDL_DECLSPEC bool SDLCALL SDL_BlitSurfaceUncheckedScaled(SDL_Surface *src
  *
  * \param src the SDL_Surface structure to be copied from.
  * \param srcrect the SDL_Rect structure representing the rectangle to be
- *                copied, may not be NULL.
+ *                copied, or NULL to copy the entire surface.
  * \param dst the SDL_Surface structure that is the blit target.
  * \param dstrect the SDL_Rect structure representing the target rectangle in
- *                the destination surface, may not be NULL.
+ *                the destination surface, or NULL to fill the entire
+ *                destination surface.
  * \param scaleMode the SDL_ScaleMode to be used.
  * \returns true on success or false on failure; call SDL_GetError() for more
  *          information.