add active model basics wip to guides/documents.yaml
support :unless_exist for FileCache
Fix indentation [ci skip]
Fix typo
fix typo in comment [ci skip].
Merge pull request #11564 from zzak/active_model_guide

[Guides] Active Model Basics [ci skip]
use a queue rather than a list. fixes #12069
Merge pull request #11785 from grosser/grosser/file-unless-exist

support :unless_exist for FileCache

Conflicts:
	activesupport/CHANGELOG.md
	activesupport/test/caching_test.rb