Added exception details to XML docs.

src/Avalonia.Controls/Window.cs

@@ -371,6 +371,9 @@ namespace Avalonia.Controls
         /// <summary>
         /// Shows the window.
         /// </summary>
+        /// <exception cref="InvalidOperationException">
+        /// The window has already been closed.
+        /// </exception>
         public override void Show()
         {
             if (PlatformImpl == null)
@@ -402,6 +405,9 @@ namespace Avalonia.Controls
         /// Shows the window as a dialog.
         /// </summary>
         /// <param name="owner">The dialog's owner window.</param>
+        /// <exception cref="InvalidOperationException">
+        /// The window has already been closed.
+        /// </exception>
         /// <returns>
         /// A task that can be used to track the lifetime of the dialog.
         /// </returns>