Add some doc

SDWebImage/SDWebImageManager.h

@@ -13,9 +13,24 @@
 
 typedef enum
 {
+    /**
+     * By default, when a URL fail to be downloaded, the URL is blacklisted so the library won't keep trying.
+     * This flag disable this blacklisting.
+     */
     SDWebImageRetryFailed = 1 << 0,
+    /**
+     * By default, image downloads are started during UI interactions, this flags disable this feature,
+     * leading to delayed download on UIScrollView deceleration for instance.
+     */
     SDWebImageLowPriority = 1 << 1,
+    /**
+     * This flag disables on-disk caching
+     */
     SDWebImageCacheMemoryOnly = 1 << 2,
+    /**
+     * This flag enables progressive download, the image is displayed progressively during download as a browser would do.
+     * By default, the image is only displayed once completely downloaded.
+     */
     SDWebImageProgressiveDownload = 1 << 3
 } SDWebImageOptions;