Add some doc

Sources/Networking/RequestModifier.swift

@@ -26,15 +26,38 @@
 
 import Foundation
 
+/// Represents and wraps a method for modifying request before an image download request starts in an asynchronous way.
 public protocol AsyncImageDownloadRequestModifier {
+
+    /// This method will be called just before the `request` being sent.
+    /// This is the last chance you can modify the image download request. You can modify the request for some
+    /// customizing purpose, such as adding auth token to the header, do basic HTTP auth or something like url mapping.
+    /// When you have done with the modification, call the `reportModified` block with the modified request and the data
+    /// download will happen with this request.
+    ///
+    /// Usually, you pass an `AsyncImageDownloadRequestModifier` as the associated value of
+    /// `KingfisherOptionsInfoItem.requestModifier` and use it as the `options` parameter in related methods.
+    ///
+    /// If you do nothing with the input `request` and return it as is, a downloading process will start with it.
+    ///
+    /// - Parameters:
+    ///   - request: The input request contains necessary information like `url`. This request is generated
+    ///              according to your resource url as a GET request.
+    ///   - reportModified: The callback block you need to call after the asynchronous modifying done.
+    ///
     func modified(for request: URLRequest, reportModified: @escaping (URLRequest?) -> Void)
+
+    /// A block will be called when the download task started.
+    ///
+    /// If an `AsyncImageDownloadRequestModifier` and the asynchronous modification happens before the download, the
+    /// related download method will not return a valid `DownloadTask` value. Instead, you can get one from this method.
     var onDownloadTaskStarted: ((DownloadTask?) -> Void)? { get }
 }
 
 /// Represents and wraps a method for modifying request before an image download request starts.
 public protocol ImageDownloadRequestModifier: AsyncImageDownloadRequestModifier {
 
-    /// A method will be called just before the `request` being sent.
+    /// This method will be called just before the `request` being sent.
     /// This is the last chance you can modify the image download request. You can modify the request for some
     /// customizing purpose, such as adding auth token to the header, do basic HTTP auth or something like url mapping.
     ///
@@ -57,6 +80,8 @@ extension ImageDownloadRequestModifier {
         reportModified(request)
     }
 
+    /// This is `nil` for a sync `ImageDownloadRequestModifier` by default. You can get the `DownloadTask` from the
+    /// return value of downloader method.
     public var onDownloadTaskStarted: ((DownloadTask?) -> Void)? { return nil }
 }