Add default value documentation for Label.LineBreakMode property (#33490)

The XML API documentation for `Label.LineBreakMode` was missing the
default value, creating inconsistency with the [documentation
site](dotnet/docs-maui#3168) which now correctly
specifies `WordWrap` as the default.

### Changes
- Added default value documentation to `LineBreakMode` property in
`Label.cs`
- Follows existing documentation pattern used for other bindable
properties (e.g., `Button.CommandParameter`, `Border.SafeAreaEdges`)

```csharp
/// <summary>
/// Gets or sets the LineBreakMode for the Label.
/// The default value is <see cref="LineBreakMode.WordWrap"/>. This is a bindable property.
/// </summary>
public LineBreakMode LineBreakMode
```

The default value `LineBreakMode.WordWrap` is confirmed in the
`BindableProperty.Create` definition at line 104.

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<details>

<summary>Original prompt</summary>

> For LineBreakMode in src/Controls/src/Core/Label/Label.cs can you
specify the default value on the API docs as also indicated here:
dotnet/docs-maui#3168


</details>



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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jfversluis <939291+jfversluis@users.noreply.github.com>

Author: Copilot

src/Controls/src/Core/Label/Label.cs

@@ -148,7 +148,10 @@ public TextAlignment HorizontalTextAlignment
 			set { SetValue(TextAlignmentElement.HorizontalTextAlignmentProperty, value); }
 		}
 
-		/// <summary>Gets or sets the LineBreakMode for the Label. This is a bindable property.</summary>
+		/// <summary>
+		/// Gets or sets the LineBreakMode for the Label.
+		/// The default value is <see cref="LineBreakMode.WordWrap"/>. This is a bindable property.
+		/// </summary>
 		public LineBreakMode LineBreakMode
 		{
 			get { return (LineBreakMode)GetValue(LineBreakModeProperty); }