Bring `after_bundle` back to API document [ci skip]

At class level `:nodoc:` all elements are prevented. Instead, use
`:stopdoc:` / `:startdoc:` to make `after_bundle` appear.

railties/lib/rails/generators/rails/app/app_generator.rb

@@ -243,7 +243,10 @@ module Generators
     # can change in Ruby 1.8.7 when we FileUtils.cd.
     RAILS_DEV_PATH = File.expand_path("../../../../../..", __dir__)
 
-    class AppGenerator < AppBase # :nodoc:
+    class AppGenerator < AppBase
+
+      # :stopdoc:
+
       WEBPACKS = %w( react vue angular elm stimulus )
 
       add_shared_options_for "application"
@@ -492,6 +495,8 @@ def self.banner
         "rails new #{arguments.map(&:usage).join(' ')} [options]"
       end
 
+      # :startdoc:
+
     private
 
       # Define file as an alias to create_file for backwards compatibility.