QDialogButtonBox::ButtonRole and QMessageBox::ButtonRole documentation is shared in the first one's cpp file. It has:

/*!
    \enum QDialogButtonBox::ButtonRole
    \enum QMessageBox::ButtonRole

    This enum describes the roles that can be used to describe buttons in
    ...
*/

The enum does appear in both classes' documentation (here and here, linked to the entry above the enum), but it doesn't have a proper title (instead, the title is Member Function Documentation), and the links to ButtonRole in both pages don't function.