Documenting errors of revoked tokens

Hi,

In our apps we need to know whether the Google token was revoked or not, so we can delete other data that was obtained using that token. Today, if one tries to call `restorePreviousSignInWithCallback:` when the token was revoked, we get the following error:

```
Error Domain=org.openid.appauth.oauth_token Code=-10 "invalid_grant: Token has been expired or revoked." UserInfo={OIDOAuthErrorResponseErrorKey={
error = "invalid_grant";
"error_description" = "Token has been expired or revoked.";
}, NSLocalizedDescription=invalid_grant: Token has been expired or revoked., NSUnderlyingError=0x281ab7e40 {Error Domain=org.openid.appauth.remote-http Code=400 "{
"error": "invalid_grant",
"error_description": "Token has been expired or revoked."
}" UserInfo={NSLocalizedDescription={
"error": "invalid_grant",
"error_description": "Token has been expired or revoked."
}}}}
```

Which comes directly from https://github.com/openid/AppAuth-iOS. Can this error be explicitly documented? Either by documenting that this is the error returned, or introducing a new error code to `GIDSignInErrorCode`.

Review the restorePreviousSignInWithCallback: behavior and the GIDSignInErrorCode API, then compare the reported invalid_grant error with the existing documentation. Done means the revoked-token error is explicitly documented, or the requested new error-code behavior is clearly addressed; the issue also references AppAuth-iOS as the source error.

Written by the indexing model from the issue text.