I think the trailing doc comment supports only a brief description (not detail), such that enums/struct members can still be one-liners. But it may be possible to add commands like @since to that, have to try (but I'd expect it should be on the same line still).
In any event, you can make full-fledged multi-line doxygen comments for members if you put it before, see [struct GeanyData](https://github.com/geany/geany/blob/master/src/plugindata.h#L170)