API is missing implementations of AuthCredential

### Environment

* Operating System version: any
* Firebase SDK version: 4.1.3
* Firebase Product: auth

### Problem

The API reference is missing details about implementations of the [`AuthCredential`](https://firebase.google.com/docs/reference/js/firebase.auth.AuthCredential) interface. There is an implementation specific to each auth provider. The attribute `accessToken`, and for Twitter also the attribute `secret` of the `AuthCredential` needs to be accessed but it is not documented in the API. The only place where it is mentioned is in the example code, for example in [GithubAuthProvider](https://firebase.google.com/docs/reference/js/firebase.auth.GithubAuthProvider):

```JavaScript
// This gives you a GitHub Access Token.
var token = result.credential.accessToken;
```

The reason I'm opening this issue is because we need to rely on the official JS API for the implementation on the [Dart side of Firebase](https://github.com/firebase/firebase-dart) to be able to resolve [this issue](https://github.com/firebase/firebase-dart/issues/118).

Start with the AuthCredential API reference and the provider references, including GithubAuthProvider, to compare the documented interface with the provider-specific implementations. Document accessToken for each implementation and Twitter's secret attribute; the API reference is complete when these properties are described rather than only shown in examples.

Written by the indexing model from the issue text.