fix(ui5-illustrated-message): improved story and documentation (#8294)

In the "Custom Title" story we had "titleLevel" story control which
didn't take any effect upon being changed. This control has been removed
and "titleLevel" property is now documented that it doesn't work when
we are using "title" slot.

packages/fiori/src/IllustratedMessage.ts

@@ -173,6 +173,10 @@ class IllustratedMessage extends UI5Element {
 	*
 	* <b>Note:</b> Used for accessibility purposes only.
 	*
+	* <br/>
+	*
+	* <b>Note:</b> Doesn't take effect when <code>title</code> slot is being used.
+	*
 	* @default "H2"
 	* @public
 	* @since 1.20.0

packages/playground/_stories/fiori/IllustratedMessage/IllustratedMessage.stories.ts

@@ -80,3 +80,7 @@ CustomTitle.args = {
     default: `
 	<ui5-button icon="refresh">Try again</ui5-button>`,
 };
+
+CustomTitle.parameters = {
+	controls: { exclude: ['titleLevel'] },
+};