Add more details around implications of setting an explicit `value` for selects

In particular, make it explicit that following lookup expressions need to use the same element to filter by.

https://docs.getodk.org/form-question-types/#customizing-the-label-and-value

Open the “Customizing the label and value” section at the linked form-question-types documentation page. Clarify the implications of setting an explicit value for selects, including that following lookup expressions must use the same element for filtering. Done means the documentation states this relationship explicitly and reads clearly in context.

Written by the indexing model from the issue text.