Document that choice labels but not choice name or other columns can have references to form data

https://github.com/XLSForm/pyxform/issues/515, https://forum.getodk.org/t/make-a-selection-from-a-roster-created-from-groups-rather-than-repeats-alternative-to-select-one-repeat/32466/10?u=ln

Start with the linked pyxform issue and ODK forum discussion, then locate the existing XLSForm choice documentation in this Sphinx repository. Document which choice columns can reference form data, distinguishing choice labels from choice names and other columns. Build the documentation to verify the change renders correctly.

Written by the indexing model from the issue text.