I'm very new to Angular so I'm still finding my way around the documentation. $compile was not a place I thought to look for information on proper usage of module.directive(...).

My search went along the line of:
(Looking for validators)
http://docs.angularjs.org/guide/forms
(Looking for more information on directive definition)
http://docs.angularjs.org/api/ng/type/angular.Module
(Link in documentation for Module.directive(...))
http://docs.angularjs.org/api/ng/provider/$compileProvider#directive
(The only link in function documentation)
http://docs.angularjs.org/guide/directive

Where the documentation states that link only takes three arguments, clearly contradicting the validator example which uses four.

I'll admit this one line might be more confusing, but it's is at least not contradictory and leaves the impression of "there is more to this but it is outside the scope of this paragraph."

As I said, I am not qualified to write more, but I feel at least a pointer in this direction would be needed.