Document the animation delay units

Fixes #550

(cherry picked from commit a8d223f)

Author: slouken

include/SDL_image.h

@@ -2039,16 +2039,17 @@ extern DECLSPEC int SDLCALL IMG_SaveJPG(SDL_Surface *surface, const char *file,
 extern DECLSPEC int SDLCALL IMG_SaveJPG_RW(SDL_Surface *surface, SDL_RWops *dst, int freedst, int quality);
 
 /**
- * Animated image support.
+ * Animated image support
  *
- * Currently only animated GIFs are supported.
+ * Currently only animated GIFs and WEBP images are supported.
  */
 typedef struct IMG_Animation
 {
-	int w, h;
-	int count;
-	SDL_Surface **frames;
-	int *delays;
+    int w;                  /**< The width of the frames */
+    int h;                  /**< The height of the frames */
+    int count;              /**< The number of frames */
+    SDL_Surface **frames;   /**< An array of frames */
+    int *delays;            /**< An array of frame delays, in milliseconds */
 } IMG_Animation;
 
 /**