asset guide, match application.rb snippets to the generated file [ci skip]

`if defined?(Bundler)` is no longer used in our current application.rb
Merge pull request #10222 from senny/update_bundler_section_in_assets_guide

docs, adjust the application.rb snippets to match the generated file.

Closes #10449
Add note about decorator loading in Engines guide.

Because decorators aren't referenced by the application, they won't get
autoloaded. And because we recommend the pattern, it would be irresponsible
to not show how to load them properly.

Fixes #10647.