select_one with map appearance : mention the need to set choice_filter to true() to show markers from choices sheet

I just created a small form with select_one and map appearance from the choices sheet.
I had to set choice_filter to true() in order to show the markers on the map (I forgot it ;-) )

Maybe this could be mentioned in the [documentation page](https://docs.getodk.org/form-question-types/#select-from-map) ?

https://github.com/getodk/docs/blob/master/docs/form-question-types.rst

Open docs/form-question-types.rst and locate the select-from-map documentation. Read the existing explanation for choices-sheet markers and document that choice_filter must be set to true() for those markers to appear. Done means the requirement is clearly stated on the linked documentation page.

Written by the indexing model from the issue text.