[Api docs] Updates ListTile.subtitle docs to address how TextStyle is decided (#61071)

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

@@ -687,6 +687,21 @@ class ListTile extends StatelessWidget {
   /// If [isThreeLine] is true, this should be configured to take a maximum of
   /// two lines. For example, you can use [Text.maxLines] to enforce the number
   /// of lines.
+  ///
+  /// The subtitle's default [TextStyle] depends on [TextTheme.bodyText2] except
+  /// [TextStyle.color]. The [TextStyle.color] depends on the value of [enabled]
+  /// and [selected].
+  ///
+  /// When [enabled] is false, the text color is set to [ThemeData.disabledColor].
+  ///
+  /// When [selected] is true, the text color is set to [ListTileTheme.selectedColor]
+  /// if it's not null. If [ListTileTheme.selectedColor] is null, the text color
+  /// is set to [ThemeData.primaryColor] when [ThemeData.brightness] is
+  /// [Brightness.light] and to [ThemeData.accentColor] when it is [Brightness.dark].
+  ///
+  /// When [selected] is false, the text color is set to [ListTileTheme.textColor]
+  /// if it's not null and to [TextTheme.caption]'s color if [ListTileTheme.textColor]
+  /// is null.
   final Widget subtitle;
 
   /// A widget to display after the title.