Update documentation for Modals (#4495)

Update documentation for modals to include the custom class introduced in #4485, as well as the footer and subtitle fields that don't appear to be documented.

editions/tw5.com/tiddlers/features/Modals.tid

@@ -1,11 +1,18 @@
 created: 20160107225427489
-modified: 20160107225651558
+modified: 20200312172056083
 tags: Features
 title: Modals
 type: text/vnd.tiddlywiki
 
 Modals (or "wizards") fade the main TiddlyWiki window to display an isolated tiddler that must be explicitly dismissed by the user.
 
+The tiddler to be displayed can contain the following optional fields that are used to customize the modal:
+
+|!Field |!Description |
+|footer|The footer text for the modal|
+|subtitle|The subtitle text for a modal|
+|class|An additional class to apply to the modal wrapper|
+
 Modals are displayed with the [[WidgetMessage: tm-modal]].
 
-<$button message="tm-modal" param="SampleWizard">Open demo modal</$button>
+<$button message="tm-modal" param="SampleWizard">Open demo modal</$button>