Add more dialog doc cross-reference (#30887)

packages/flutter/lib/src/cupertino/dialog.dart

@@ -125,6 +125,7 @@ bool _isInAccessibilityMode(BuildContext context) {
 ///  * [CupertinoPopupSurface], which is a generic iOS-style popup surface that
 ///    holds arbitrary content to create custom popups.
 ///  * [CupertinoDialogAction], which is an iOS-style dialog button.
+///  * [AlertDialog], a Material Design alert dialog.
 ///  * <https://developer.apple.com/ios/human-interface-guidelines/views/alerts/>
 class CupertinoAlertDialog extends StatelessWidget {
   /// Creates an iOS-style alert dialog.

packages/flutter/lib/src/material/dialog.dart

@@ -189,6 +189,7 @@ class Dialog extends StatelessWidget {
 ///
 ///  * [SimpleDialog], which handles the scrolling of the contents but has no [actions].
 ///  * [Dialog], on which [AlertDialog] and [SimpleDialog] are based.
+///  * [CupertinoAlertDialog], an iOS-styled alert dialog.
 ///  * [showDialog], which actually displays the dialog and returns its result.
 ///  * <https://material.io/design/components/dialogs.html#alert-dialog>
 class AlertDialog extends StatelessWidget {