Comment to make nil resource behavior clear

Sources/ImageView+Kingfisher.swift

@@ -49,6 +49,9 @@ extension Kingfisher where Base: ImageView {
      
      - note: Both the `progressBlock` and `completionHandler` will be invoked in main thread.
      The `CallbackDispatchQueue` specified in `optionsInfo` will not be used in callbacks of this method.
+     
+     If `resource` is `nil`, the `placeholder` image will be set and
+     `completionHandler` will be called with both `error` and `image` being `nil`.
      */
     @discardableResult
     public func setImage(with resource: Resource?,

Sources/NSButton+Kingfisher.swift

@@ -45,6 +45,9 @@ extension Kingfisher where Base: NSButton {
      
      - note: Both the `progressBlock` and `completionHandler` will be invoked in main thread.
      The `CallbackDispatchQueue` specified in `optionsInfo` will not be used in callbacks of this method.
+     
+     If `resource` is `nil`, the `placeholder` image will be set and
+     `completionHandler` will be called with both `error` and `image` being `nil`.
      */
     @discardableResult
     public func setImage(with resource: Resource?,
@@ -115,6 +118,9 @@ extension Kingfisher where Base: NSButton {
      
      - note: Both the `progressBlock` and `completionHandler` will be invoked in main thread.
      The `CallbackDispatchQueue` specified in `optionsInfo` will not be used in callbacks of this method.
+     
+     If `resource` is `nil`, the `placeholder` image will be set and
+     `completionHandler` will be called with both `error` and `image` being `nil`.
      */
     @discardableResult
     public func setAlternateImage(with resource: Resource?,

Sources/UIButton+Kingfisher.swift

@@ -46,6 +46,9 @@ extension Kingfisher where Base: UIButton {
      
      - note: Both the `progressBlock` and `completionHandler` will be invoked in main thread.
      The `CallbackDispatchQueue` specified in `optionsInfo` will not be used in callbacks of this method.
+     
+     If `resource` is `nil`, the `placeholder` image will be set and
+     `completionHandler` will be called with both `error` and `image` being `nil`.
      */
     @discardableResult
     public func setImage(with resource: Resource?,
@@ -121,6 +124,9 @@ extension Kingfisher where Base: UIButton {
      
      - note: Both the `progressBlock` and `completionHandler` will be invoked in main thread.
      The `CallbackDispatchQueue` specified in `optionsInfo` will not be used in callbacks of this method.
+     
+     If `resource` is `nil`, the `placeholder` image will be set and
+     `completionHandler` will be called with both `error` and `image` being `nil`.
      */
     @discardableResult
     public func setBackgroundImage(with resource: Resource?,