When editing a relationship attribute such as https://musicbrainz.org/relationship-attribute/a40b43ed-2722-4b4a-98a5-478283cdf8df a relationship editor can modify two checkboxes: "This attribute supports free text credits" and "This attribute uses free text values". The value of these is not actually shown in any way in the main attribute page though AFAICT, meaning non-admins have no way of telling from the docs whether a specific attribute supports free text in any way.