Update documentation of ASNetworkImageNodeDelegate #trivial (#1163)

`-imageNodeDidStartFetchingData:` and `-imageNodeDidFinishDecoding:` are always called on main thread, and the documentation should reflect that.

Source/ASNetworkImageNode.h

@@ -145,6 +145,15 @@ NS_ASSUME_NONNULL_BEGIN
 @protocol ASNetworkImageNodeDelegate <NSObject>
 @optional
 
+/**
+ * Notification that the image node started to load
+ *
+ * @param imageNode The sender.
+ *
+ * @discussion Called on the main thread.
+ */
+- (void)imageNodeDidStartFetchingData:(ASNetworkImageNode *)imageNode;
+
 /**
  * Notification that the image node finished downloading an image, with additional info.
  * If implemented, this method will be called instead of `imageNode:didLoadImage:`.
@@ -167,15 +176,6 @@ NS_ASSUME_NONNULL_BEGIN
  */
 - (void)imageNode:(ASNetworkImageNode *)imageNode didLoadImage:(UIImage *)image;
 
-/**
- * Notification that the image node started to load
- *
- * @param imageNode The sender.
- *
- * @discussion Called on a background queue.
- */
-- (void)imageNodeDidStartFetchingData:(ASNetworkImageNode *)imageNode;
-
 /**
  * Notification that the image node failed to download the image.
  *
@@ -190,6 +190,8 @@ NS_ASSUME_NONNULL_BEGIN
  * Notification that the image node finished decoding an image.
  *
  * @param imageNode The sender.
+ *
+ * @discussion Called on the main thread.
  */
 - (void)imageNodeDidFinishDecoding:(ASNetworkImageNode *)imageNode;