Document that new surfaces are initialized to zero.

Fixes https://github.com/libsdl-org/SDL/issues/10577

include/SDL3/SDL_surface.h

@@ -125,6 +125,8 @@ typedef struct SDL_Surface
 /**
  * Allocate a new surface with a specific pixel format.
  *
+ * The pixels of the new surface are initialized to zero.
+ *
  * \param width the width of the surface.
  * \param height the height of the surface.
  * \param format the SDL_PixelFormat for the new surface's pixel format.