[FR] docs: Please update comments in source code.

[FrancescoJo]

Is your feature request related to a problem? Please describe.
https://github.com/firebase/firebase-admin-node/blob/master/src/auth/index.ts#L503

Now in december 2020, Firebase auth supports various third pary login methods more than anonymous, password, facebook.com, github.com, google.com, twitter.com, or custom. However, only all these methods that I listed are written in source code comments, which causes confusion.

Of course enumerating all possible tokens like this way is way much better than other sdks(e.g.: Firebase Java sdk), but it needs to be updated. At least in my case, I can confirm there are more providerIds are returned by auth API, such as apple.com or microsoft.com.

So this is my little proposal: could you guys update the comments to elaborate all possible providerIds please?

Describe the solution you'd like
Comment includes all possible providerId candidates: not in Java sdk way

Describe alternatives you've considered
Or sync to Java sdk way which I think the worst solution but it would decrease confusion

Additional context
n/a

Thanks for reading.

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