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.
Fix some typo in method names, variables
compatability => compatibility
Merge pull request #10761 from vipulnsward/fix-typo-1

Fix some typo in method names, variables
Merge pull request #10762 from vipulnsward/change_compatibility

compatability => compatibility
add test-case for `Array#to_sentence with blank items.
Merge pull request #10763 from senny/10758_to_sentence_with_blank_elements

test-case for `Array#to_sentence` with `blank?` items.