Add info about Page.OnAppearing. (#2368)

* Add info about Page.OnAppearing.

* Edit.

docs/fundamentals/windows.md

@@ -37,7 +37,7 @@ The <xref:Microsoft.Maui.Controls.Window> class defines the following events:
 - <xref:Microsoft.Maui.Controls.Window.Destroying>, which is raised when the window is destroyed.
 - <xref:Microsoft.Maui.Controls.Window.SizeChanged>, which is raised on desktop platforms when the window changes size.
 - <xref:Microsoft.Maui.Controls.Window.Backgrounding>, with an accompanying `BackgroundingEventArgs` object, which is raised on iOS and Mac Catalyst when the window is closed or enters a background state. This event can be used to persist any `string` state to the `State` property of the `BackgroundingEventArgs` object, which the OS will preserve until it's time to resume the window. When the window is resumed the state is provided via the `IActivationState` argument to the `CreateWindow` method.
-- <xref:Microsoft.Maui.Controls.Window`DisplayDensityChanged`, with an accompanying `DisplayDensityChangedEventArgs` object, which is raised on Android and Windows when the effective dots per inch (DPI) for the window has changed.
+- <xref:Microsoft.Maui.Controls.Window.DisplayDensityChanged>, with an accompanying `DisplayDensityChangedEventArgs` object, which is raised on Android and Windows when the effective dots per inch (DPI) for the window has changed.
 
 For more information about the lifecycle events, and their associated overrides, see [App lifecycle](app-lifecycle.md).
 

docs/migration/includes/api-changes.md

@@ -149,6 +149,8 @@ A small number of other APIs have been consolidated in the move from Xamarin.For
 > | `Xamarin.Forms.Span.ForegroundColor` | <xref:Microsoft.Maui.Controls.Span.TextColor?displayProperty=fullName> |  |
 > | `Xamarin.Forms.ToolbarItem.Name` | <xref:Microsoft.Maui.Controls.MenuItem.Text?displayProperty=fullName> | <xref:Microsoft.Maui.Controls.MenuItem.Text?displayProperty=fullName> is the base class for <xref:Microsoft.Maui.Controls.ToolbarItem?displayProperty=fullName>, and so `ToolbarItem.Name` becomes `ToolbarItem.Text`. |
 
+In addition, in Xamarin.Forms, the `Page.OnAppearing` override is called on Android when an app is backgrounded and then brought to the foreground. However, this override isn't called on iOS and Windows in the same scenario. In .NET MAUI, the <xref:Microsoft.Maui.Controls.Page.OnAppearing> override isn't called on any platforms when an app is backgrounded and then brought to the foreground. Instead, you should listen to lifecycle events on <xref:Microsoft.Maui.Controls.Window> to be notified when an app returns to the foreground. For more information, see [.NET MAUI windows](~/fundamentals/windows.md).
+
 ### Native forms changes
 
 [Native forms](/xamarin/xamarin-forms/platform/native-forms) in Xamarin.Forms has become native embedding in .NET MAUI, and uses a different initialization approach and different extension methods to convert cross-platform controls to their native types. For more information, see [Native embedding](~/platform-integration/native-embedding.md).