Add dart doc

tool/build_e2e.dart

@@ -4,13 +4,23 @@
 
 import 'dart:io';
 
-const argDevToolsBuild = 'devtools-build';
-const argUpdatePerfetto = '--update-perfetto';
-const argNoUpdateFlutter = '--no-update-flutter';
-
-// Add this flag if devtools_app does not need to be rebuilt.
 const argNoBuildApp = '--no-build-app';
+const argNoUpdateFlutter = '--no-update-flutter';
+const argUpdatePerfetto = '--update-perfetto';
 
+/// This script builds DevTools in release mode by running the
+/// `./tool/build_release.sh` script and then serves DevTools with a locally
+/// running DevTools server.
+/// 
+/// If [argNoBuildApp] is present, the DevTools web app will not be rebuilt.
+/// 
+/// If [argNoUpdateFlutter] is present, the Flutter SDK will not be updated to
+/// the latest Flutter candidate. Use this flag to save the cost of updating the
+/// Flutter SDK when you already have the proper SDK checked out.
+/// 
+/// If [argUpdatePerfetto] is present, the precompiled bits for Perfetto will
+/// be updated from the [update_perfetto.sh] script as part of the DevTools
+/// build process (e.g. [build_release.sh]).
 void main(List<String> args) async {
   final shouldUpdatePerfetto = args.contains(argUpdatePerfetto);
   final noUpdateFlutter = args.contains(argNoUpdateFlutter);