Supported locales docs to explain proper scriptCode usage. (#24184)

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

@@ -560,6 +560,34 @@ class WidgetsApp extends StatefulWidget {
   /// [supportedLocales] with a matching [Locale.languageCode] is used. If that
   /// fails then the first locale in [supportedLocales] is used. The default
   /// locale resolution algorithm can be overridden with [localeResolutionCallback].
+  ///
+  /// When supporting languages with more than one script, it is recommended
+  /// to specify the [Locale.scriptCode] explicitly. Locales may also be defined without
+  /// [Locale.countryCode] to specify a generic fallback for a particular script.
+  ///
+  /// A fully supported language with multiple scripts should define a generic language-only
+  /// locale (eg. 'zh'), language+script only locales (eg. 'zh_Hans' and 'zh_Hant'),
+  /// and any language+script+country locales (eg. 'zh_Hans_CN'). Fully defining all of
+  /// these locales as supported is not strictly required but allows for proper locale resolution in
+  /// the most number of cases. These locales can be specified with the [Locale.fromSubtags]
+  /// constructor:
+  ///
+  /// ```dart
+  /// // Full Chinese support for CN, TW, and HK
+  /// supportedLocales: [
+  ///   const Locale.fromSubtags(languageCode: 'zh'), // generic Chinese 'zh'
+  ///   const Locale.fromSubtags(languageCode: 'zh', scriptCode: 'Hans'), // generic simplified Chinese 'zh_Hans'
+  ///   const Locale.fromSubtags(languageCode: 'zh', scriptCode: 'Hant'), // generic traditional Chinese 'zh_Hant'
+  ///   const Locale.fromSubtags(languageCode: 'zh', scriptCode: 'Hans', countryCode: 'CN'), // 'zh_Hans_CN'
+  ///   const Locale.fromSubtags(languageCode: 'zh', scriptCode: 'Hant', countryCode: 'TW'), // 'zh_Hant_TW'
+  ///   const Locale.fromSubtags(languageCode: 'zh', scriptCode: 'Hant', countryCode: 'HK'), // 'zh_Hant_HK'
+  /// ],
+  /// ```
+  ///
+  /// Omitting some these fallbacks may result in improperly resolved
+  /// edge-cases, for example, a simplified Chinese user in Taiwan ('zh_Hans_TW')
+  /// may resolve to traditional Chinese if 'zh_Hans' and 'zh_Hans_CN' are
+  /// omitted.
   /// {@endtemplate}
   ///
   /// See also: