Add +variants to ActionView::FileSystemResolver documentation.
Merge pull request #17070 from felipecvo/master

Add +variants to ActionView::FileSystemResolver documentation.
Don’t require already required modules

abstract_unit.rb requires `action_controller` which [already includes the following lines of code](https://github.com/rails/rails/blob/64fcdce1d3a6a8768ab17f3be144270456814f82/actionpack/lib/action_controller.rb#L2L3):

```ruby
require 'abstract_controller'
require 'action_dispatch'
```
Merge pull request #17071 from claudiob/don-t-require-action-dispatch-twice

Don’t require already required modules
Preparing for 4.2.0.beta2 release