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In our apps we need to know whether the Google token was revoked or not, so we can delete other data that was obtained using that token. Today, if one tries to call `restorePreviousSignInWithCallback:` when the token was revoked, we get the following error: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