Add note about support for numbers vs Enums vs strings
Add note about support for numbers vs Enums vs strings

+ For context, see: https://github.com/openfisca/openfisca-core/issues/938

openfisca/openfisca-doc,openfisca/openfisca-doc,openfisca/openfisca-doc,openfisca/openfisca-doc