Fix item action guide

[krns]

[Flo0807]

Currently, the item action confirmation dialog is enabled when the list of fields is empty or the confirm/1 function is exported (see has_modal?/1).

From our docs:

To enable the confirmation dialog you need to implement the confirm_label/1 function

This doesn't seem right and has to be updated.

Moreover, we should update the has_modal?/1 function because you always need the confirm/1 function if there is a list of fields.

@krns

[Flo0807]

See https://github.com/naymspace/backpex/pull/760