Small glow overscroll dartdoc addition (#14579)

* Small glow overscroll dartdoc addition

* Directly specify [MaterialApp]

* review

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

@@ -306,7 +306,7 @@ class ThemeData {
   /// icons placed on top of the primary color (e.g. toolbar text).
   final Brightness primaryColorBrightness;
 
-  /// The foreground color for widgets (knobs, text, etc)
+  /// The foreground color for widgets (knobs, text, overscroll edge effect, etc).
   final Color accentColor;
 
   /// The brightness of the [accentColor]. Used to determine the color of text

packages/flutter/lib/src/widgets/overscroll_indicator.dart

@@ -30,6 +30,8 @@ import 'ticker_provider.dart';
 ///
 /// Created automatically by [ScrollBehavior.buildViewportChrome] on platforms
 /// (e.g., Android) that commonly use this type of overscroll indication.
+///
+/// In a [MaterialApp], the edge glow color is the [ThemeData.accentColor].
 class GlowingOverscrollIndicator extends StatefulWidget {
   /// Creates a visual indication that a scroll view has overscrolled.
   ///

packages/flutter/lib/src/widgets/scroll_physics.dart

@@ -362,7 +362,8 @@ class BouncingScrollPhysics extends ScrollPhysics {
 ///    behavior.
 ///  * [GlowingOverscrollIndicator], which is used by [ScrollConfiguration] to
 ///    provide the glowing effect that is usually found with this clamping effect
-///    on Android.
+///    on Android. When using a [MaterialApp], the [GlowingOverscrollIndicator]'s
+///    glow color is specified to use [ThemeData.accentColor].
 class ClampingScrollPhysics extends ScrollPhysics {
   /// Creates scroll physics that prevent the scroll offset from exceeding the
   /// bounds of the content..