Document default appearance for selects with images in Collect

The default appearance (`""`) when using selects with images shows them as "thumbnails" rather than filling the available space:


This is different from the [`columns-n` appearance](https://docs.getodk.org/form-question-types/#select-widget-with-columns-n-appearance) which might be confusing. We should make sure it's clear how the default appearance behaves that it is different from `columns-1`.

Start with the linked documentation section for the select widget with the columns-n appearance and compare it with the default appearance described in the issue. Update the documentation so readers can tell that the default select-with-images behavior shows thumbnails and differs from columns-1; done when that distinction is clear.

Written by the indexing model from the issue text.