Update the comments

SDWebImage/Core/SDWebImageDefine.h

@@ -181,6 +181,7 @@ typedef NS_OPTIONS(NSUInteger, SDWebImageOptions) {
     /**
      * By default, when you use `SDWebImageContextAnimatedImageClass` context option (like using `SDAnimatedImageView` which designed to use `SDAnimatedImage`), we may still use `UIImage` when the memory cache hit, or image decoder is not available to produce one exactlly matching your custom class as a fallback solution.
      * Using this option, can ensure we always callback image with your provided class. If failed to produce one, a error with code `SDWebImageErrorBadImageData` will been used.
+     * Note this options is not compatible with `SDWebImageDecodeFirstFrameOnly`, which always produce a UIImage/NSImage.
      */
     SDWebImageMatchAnimatedImageClass = 1 << 21,
 };