Documentation on Angular Material $mdDialog.finally?

From the documentation, dialog boxes in Angular Material have a signature like so:

function showAlert() {
  alert = $mdDialog.alert()
    .title('Attention, ' + $scope.userName)
    .content('This is an example of how easy dialogs can be!')
    .ok('Close');
  $mdDialog
      .show( alert )
      .finally(function() {
        alert = undefined;
      });
}

I can't seem to find any documentation on .finally. It appears to be a callback function from what I can gather, though the documentation is oddly lacking any info.

Should I assume it is a normal callback function—and why is the documentation on it lacking—is this simply such standard directive syntax that this is assumed to be the way to deal with callbacks, like .then?

Thanks for any information.