Usage part for directives with boolean/void attribute selectors

[Narretz]

The usage section renders the possible combinations based on the params (=attributes). When an attribute directive does not use the attribute value, it usually has no param defined for that either, which means the attribute is missing in the usage section.
I think this should be special cased in here https://github.com/angular/dgeni-packages/blob/master/ngdoc/templates/api/directive.template.html