Update NC docs to reflect accurate authentication details

The [Forming Node Classifier Requests](https://docs.puppetlabs.com/pe/latest/nc_forming_requests.html) documentation is currently inaccurate. 

* There is currently no concept in RBAC of associating a certificate with a user, and so the linked documentation of course provides no actual further info.
* To my knowledge, there's not really any sane way of disabling RBAC in PE, so implying that there is one probably isn't wise.

I've added the actual steps to get authentication against the NC API working, along with an example `curl` query that customers can look at and modify.