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

[mathieubossaert]

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