Update External-Commands docs for new command abstraction

docs/External-Commands.md

@@ -56,16 +56,19 @@ External commands can be hosted in a [tap](Taps.md) to allow users to easily ins
 
 External commands should be added to a `cmd` directory in the tap. An external command `extcmd` implemented as a Ruby command should live in `cmd/extcmd.rb` (don't forget to `chmod +x`).
 
-To easily use Homebrew's argument parser, replicate the following Ruby template for external commands (replacing all instances of `foo` with the name of the command):
+To easily use Homebrew's argument parser, replicate the Ruby template below for external commands. Your implementation must include the following:
+
+- The class name should be the command name in CamelCase (e.g. `my-cmd` should be named `MyCmd`).
+- Provide a `cmd_args` block that describes the command and its arguments.
+- Implement the `run` method, which will be invoked when the command is executed. Within the `run` method, the parsed arguments are available asusing `args`.
 
 ```ruby
 # frozen_string_literal: true
 
 module Homebrew
-  module_function
-
-  def foo_args
-    Homebrew::CLI::Parser.new do
+  module Cmd
+    class Foo < AbstractCommand
+      cmd_args do
         description <<~EOS
           Do something. Place a description here.
         EOS
@@ -78,15 +81,14 @@ module Homebrew
 
         named_args [:formula, :cask], min: 1
       end
-  end
-
-  def foo
-    args = foo_args.parse
 
+      def run
         something if args.force?
         something_else if args.file == "file.txt"
       end
     end
+  end
+end
 ```
 
 Using the above will generate appropriate help text: