Document the packed integer formats SDL_Color is bit-compatible with.

include/SDL_pixels.h

@@ -301,6 +301,11 @@ typedef enum
         SDL_DEFINE_PIXELFOURCC('O', 'E', 'S', ' ')
 } SDL_PixelFormatEnum;
 
+/**
+ * The bits of this structure can be directly reinterpreted as an integer-packed
+ * color which uses the SDL_PIXELFORMAT_RGBA32 format (SDL_PIXELFORMAT_ABGR8888
+ * on little-endian systems and SDL_PIXELFORMAT_RGBA8888 on big-endian systems).
+ */
 typedef struct SDL_Color
 {
     Uint8 r;