Fix callback info for showSaveDialog and showOpenDialog (electron#15549)

Author: nixxquality

docs/api/dialog.md

@@ -52,8 +52,8 @@ The `dialog` module has the following methods:
     boxes.
   * `securityScopedBookmarks` Boolean (optional) _masOS_ _mas_ - Create [security scoped bookmarks](https://developer.apple.com/library/content/documentation/Security/Conceptual/AppSandboxDesignGuide/AppSandboxInDepth/AppSandboxInDepth.html#//apple_ref/doc/uid/TP40011183-CH3-SW16) when packaged for the Mac App Store.
 * `callback` Function (optional)
-  * `filePaths` String[] - An array of file paths chosen by the user
-  * `bookmarks` String[] _macOS_ _mas_ - An array matching the `filePaths` array of base64 encoded strings which contains security scoped bookmark data. `securityScopedBookmarks` must be enabled for this to be populated.
+  * `filePaths` String[] (optional) - An array of file paths chosen by the user. If the dialog is cancelled this will be `undefined`.
+  * `bookmarks` String[] (optional) _macOS_ _mas_ - An array matching the `filePaths` array of base64 encoded strings which contains security scoped bookmark data. `securityScopedBookmarks` must be enabled for this to be populated.
 
 Returns `String[]`, an array of file paths chosen by the user,
 if the callback is provided it returns `undefined`.
@@ -103,8 +103,8 @@ shown.
     defaults to `true`.
   * `securityScopedBookmarks` Boolean (optional) _masOS_ _mas_ - Create a [security scoped bookmark](https://developer.apple.com/library/content/documentation/Security/Conceptual/AppSandboxDesignGuide/AppSandboxInDepth/AppSandboxInDepth.html#//apple_ref/doc/uid/TP40011183-CH3-SW16) when packaged for the Mac App Store. If this option is enabled and the file doesn't already exist a blank file will be created at the chosen path.
 * `callback` Function (optional)
-  * `filename` String
-  * `bookmark` String _macOS_ _mas_ - Base64 encoded string which contains the security scoped bookmark data for the saved file. `securityScopedBookmarks` must be enabled for this to be present.
+  * `filename` String (optional) If the dialog is cancelled this will be `undefined`.
+  * `bookmark` String (optional) _macOS_ _mas_ - Base64 encoded string which contains the security scoped bookmark data for the saved file. `securityScopedBookmarks` must be enabled for this to be present.
 
 Returns `String`, the path of the file chosen by the user,
 if a callback is provided it returns `undefined`.