Added selectable_column to the beginning of the batch action docs
Added redirect_to to the create your own batch action example
Merge pull request #2496 from tomprats/batch-actions-docs

Update batch actions docs
Remove IH overrrides

Now the shorthand syntax works:
```ruby
controller do
  # Redirects to index page instead of rendering udpated resource
  def update
    update!{ collection_path }
  end
end
```