Document how to customize Field::Select option labels

[cabe56]

Customizing select dropdowns helps keep the UI friendly without having to store lengthy values in a database column.

After looking through the customizing_dashboard docs I did not find a reference on how to do this. These resources helped me figure out how the select field's _form view handled option labels:

• https://github.com/thoughtbot/administrate/blob/main/app/views/fields/select/_form.html.erb
• https://apidock.com/rails/ActionView/Helpers/FormOptionsHelper/options_from_collection_for_select

These are before/after screenshots of this PR:

Documenting support for this would imply supporting [[value, label]] as an input for :collection in Field::Select.with_options. If this is a concern I can write some tests to ensure this is accounted for in future releases.

[pablobm]

Uh, good point. I agree we should document this. I'm thinking that perhaps we should mention that it's the same options that would be passed to options_from_collection_for_select (as you correctly point out) with a link to its documentation. What do you think?

[pablobm]

Actually, I'm wrong. The code does assume things, with that :first/:last in the array case... OK. Let's leave it as you propose 👍