Add docs about horizontal scrollbars (#97673)

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

@@ -723,6 +723,10 @@ class MaterialApp extends StatefulWidget {
 /// When using the desktop platform, if the [Scrollable] widget scrolls in the
 /// [Axis.vertical], a [Scrollbar] is applied.
 ///
+/// If the scroll direction is [Axis.horizontal] scroll views are less
+/// discoverable, so consider adding a Scrollbar in these cases, either directly
+/// or through the [buildScrollbar] method.
+///
 /// [MaterialScrollBehavior.androidOverscrollIndicator] specifies the
 /// overscroll indicator that is used on [TargetPlatform.android]. When null,
 /// [ThemeData.androidOverscrollIndicator] is used. If also null, the default

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

@@ -47,6 +47,10 @@ const Duration _kScrollbarTimeToFade = Duration(milliseconds: 600);
 /// without the fade animation. This requires that a [ScrollController] is
 /// provided to controller, or that the [PrimaryScrollController] is available.
 ///
+/// When a [ScrollView.scrollDirection] is [Axis.horizontal], it is recommended
+/// that the [Scrollbar] is always visible, since scrolling in the horizontal
+/// axis is less discoverable.
+///
 /// ** See code in examples/api/lib/material/scrollbar/scrollbar.1.dart **
 /// {@end-tool}
 ///