Updated instructions of 'flutter help attach' (#61569)

packages/flutter_tools/doc/attach.md

@@ -7,7 +7,7 @@ without `flutter run` and provides a HotRunner (enabling hot reload/restart).
 
 ## Usage
 
-There are four ways for the attach command to discover a running app:
+There are three ways for the attach command to discover a running app:
 
 1. If the platform is Fuchsia the module name must be provided, e.g. `$
 flutter attach --module=mod_name`. This can be called either before or after

packages/flutter_tools/lib/src/commands/attach.dart

@@ -111,7 +111,20 @@ class AttachCommand extends FlutterCommand {
   final String name = 'attach';
 
   @override
-  final String description = 'Attach to a running application.';
+  final String description = '''Attach to a running application.
+
+  For attaching to Android or iOS devices, simply using `flutter attach` is
+  usually sufficient. The tool will search for a running Flutter app or module,
+  if available. Otherwise, the tool will wait for the next Flutter app or module
+  to launch before attaching.
+
+  For Fuchsia, the module name must be provided, e.g. `\$flutter attach
+  --module=mod_name`. This can be called either before or after the application
+  is started.
+
+  If the app or module is already running and the specific observatory port is
+  known, it can be explicitly provided to attach via the command-line, e.g.
+  `\$ flutter attach --debug-port 12345`''';
 
   int get debugPort {
     if (argResults['debug-port'] == null) {