Move category example explanations into `_description_example.detail` attribute

[vaitkus]

The dictionary provides many detailed examples in category definitions. These examples are mostly formatted as partial CIF files and could thus even be automatically validated to some extent. However, currently most of these example end with a free-text footer, that describes the example, e.g.:

        #======================================================================

        Example based on the article:
        Laura Canadillas-Delgado, Lidia Mazzuca, Oscar Fabelo and
        Juan Rodriguez-Carvajal (2019) IUCrJ. 6, 105-115.
        doi:10.1107/S2052252518015026
        Incommensurate structures of the [CH~3~NH~3~][Co(COOH)~3~] compound

It might be better to move these footer explanations to the _description_example.detail attribute or at least prefix the them with the CIF comment symbol (#).