Add what type of token is returned

Link to google oauth2 cloud identity page

Debating if https://firebase.google.com/docs/auth#how_does_it_work is required because it contains "You can also use the provided authentication token to verify the identity of users in your own backend services." in the second paragraph.

Change token type to refresh token

README.md

@@ -109,7 +109,7 @@ The Firebase CLI requires a browser to complete authentication, but is fully
 compatible with CI and other headless environments.
 
 1. On a machine with a browser, install the Firebase CLI.
-2. Run `firebase login:ci` to log in and print out a new access token
+2. Run `firebase login:ci` to log in and print out a new [refresh token](https://developers.google.com/identity/protocols/OAuth2)
    (the current CLI session will not be affected).
 3. Store the output token in a secure but accessible way in your CI system.