Explain how creating a phone session can create a new user

[gewenyu99]

What does this PR do?

Attempt to clarify registration through phone authentication.

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[gewenyu99]

I wanted to discuss the appropriateness of adding a section somewhere in the quoted Account API described below. I think it would be useful to mention all the different ways a new account can be created, in a single sentence. I wonder if this belongs here, or if we should accelerate our progress on working on a Auth guide of sorts.

The Account service allows you to authenticate and manage a user account. You can use the account service to update user information, retrieve the user sessions across different devices, and fetch the user security logs with his or her recent activity.

You can authenticate the user account by using multiple sign-in methods available. Once the user is authenticated, a new session object will be created to allow the user to access his or her private data and settings.