doxygen: support brief for enum values.

doxygen: support brief for enum values.

Turns out the enum values can be documented like this:

    //! A value
    enum Enum {
        Value, //!< A value
    };

and the /// or //! comments, unlike /** */, are treated as brief by
doxygen. This is a completely valid use case and I don't want to force
users to a different syntax here, so here ya go.