Document the default value for TreeViewItem.IsExpanded

[Copilot]

This updates the TreeViewItem.IsExpanded API reference to include its default value, addressing the missing default-value guidance called out for XAML property documentation.

• API reference update

  • Added the default value to the ## -property-value section of TreeViewItem.IsExpanded.
  • Matched the wording pattern already used by similar XAML property docs such as NavigationViewItem.IsExpanded.

• Result

  • The property value text now explicitly states that the default is false.

**true** if the tree node is expanded; otherwise, **false**. The default value is **false**.

• Fixes XAML properties' default values #99