Document MaterialApp's _errorTextStyle (#12842)

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

@@ -14,6 +14,13 @@ import 'material_localizations.dart';
 import 'page.dart';
 import 'theme.dart';
 
+/// [MaterialApp] uses this [TextStyle] as its [DefaultTextStyle] to encourage
+/// developers to be intentional about their [DefaultTextStyle].
+///
+/// In Material Design, most [Text] widgets are contained in [Material] widgets,
+/// which sets a specific [DefaultTextStyle]. If you're seeing text that uses
+/// this text style, consider putting your text in a [Material] widget (or
+/// another widget that sets a [DefaultTextStyle]).
 const TextStyle _errorTextStyle = const TextStyle(
   color: const Color(0xD0FF0000),
   fontFamily: 'monospace',
@@ -21,7 +28,8 @@ const TextStyle _errorTextStyle = const TextStyle(
   fontWeight: FontWeight.w900,
   decoration: TextDecoration.underline,
   decorationColor: const Color(0xFFFFFF00),
-  decorationStyle: TextDecorationStyle.double
+  decorationStyle: TextDecorationStyle.double,
+  debugLabel: 'fallback style; consider putting your text in a Material',
 );
 
 /// An application that uses material design.