BottomAppBar doc: Fix doc about surfaceTintColor (#124563)

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

@@ -127,11 +127,17 @@ class BottomAppBar extends StatefulWidget {
   /// Not used if [shape] is null.
   final double notchMargin;
 
-  /// The color used as an overlay on [color] to indicate elevation.
+  /// A custom color for the Material 3 surface-tint elevation effect.
   ///
-  /// If this is null, no overlay will be applied. Otherwise the
-  /// color will be composited on top of [color] with an opacity related
-  /// to [elevation] and used to paint the background of the [BottomAppBar].
+  /// In Material 3, a "surface tint" with an opacity related to [elevation]
+  /// will be applied to the [BottomAppBar]'s background.
+  /// Use this property to override the default color of that tint.
+  ///
+  /// If this property is null, then [BottomAppBarTheme.surfaceTintColor]
+  /// of [ThemeData.bottomAppBarTheme] is used.
+  /// If that is also null, [ColorScheme.surfaceTint] is used.
+  ///
+  /// Ignored if [ThemeData.useMaterial3] is false.
   ///
   /// The default is null.
   ///