Add flutter attach documentation (#31279)

* add attach.md document

packages/flutter_tools/doc/attach.md

+# Flutter Attach
+
+## Overview
+
+A Flutter-command that attaches to applications that have been launched
+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:
+
+1. If the app is already running and the observatory port is known, it can be
+explicitly provided to attach via the command-line, e.g. `$ flutter attach
+--debug-port 12345`
+1. If the app is already running and the platform is iOS, attach can use mDNS
+to lookup the observatory port via the application ID, with just `$ flutter
+attach`
+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
+the application is started, attach will poll the device if it cannot
+immediately discover the port
+1. On other platforms (i.e. Android), if the app is not yet running attach
+will listen and wait for the app to be (manually) started with the default
+command: `$ flutter attach`
+
+## Source
+
+See the [source](https://github.com/flutter/flutter/blob/master/packages/flutter_tools/lib/src/commands/attach.dart) for the attach command.