content: API reference for NavController provider

_Original issue by @PeterHdd on [2019-02-28T13:19:24Z](https://github.com/ionic-team/ionic/issues/17639)_

# Documentation

I imported the following:

`import { NavController } from '@ionic/angular';`

and when checking the methods I found that it contains: `navigateBack`, `navigateRoot`, and `navigateForward`.. but I can't find these methods in the documentation.

In the `ion-nav` api: https://ionicframework.com/docs/api/nav I found different methods, not sure if this is a different class other than `navcontroller`. I would appreciate if someone can just clear this to me.

Thank you!

Start with the existing ion-nav API page and the NavController API exposed by @ionic/angular. Compare the documented ion-nav methods with navigateBack, navigateRoot, and navigateForward, then clarify the distinction and add the missing NavController method reference. Done means users can find these methods and understand which class they belong to.

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