i18n [nfc]: Add doc on GlobalLocalizations.zulipLocalizations

Author: gnprice

lib/model/localizations.dart

@@ -1,6 +1,17 @@
 import '../generated/l10n/zulip_localizations.dart';
 
 abstract final class GlobalLocalizations {
+  /// The [ZulipLocalizations] for the user's chosen language and locale.
+  ///
+  /// Where possible, the [ZulipLocalizations] should be acquired
+  /// through [ZulipLocalizations.of] instead, using a [BuildContext].
+  /// This static field is to be used where access to a [BuildContext]
+  /// is impractical, such as in the API bindings
+  /// (which use localizations when throwing exceptions).
+  ///
+  /// This gets set during app startup once we have the user's choice of locale.
+  /// If accessed before that point, it uses the app's first supported locale,
+  /// namely 'en'.
   static ZulipLocalizations zulipLocalizations =
     lookupZulipLocalizations(ZulipLocalizations.supportedLocales.first);
 }