Docs should be updated to include the extended return value for the oauth_cb callback

[ethaden]

Description

Currently, the docs to not mention that a method configured as callback for oauth_cb can return two optional values in addition to the required once. Instead of "(token_str, expiry_time)", the return value would looks something like this now: "(token_str, expiry_time [, prinicpal, dict_of_extension_key_value_pairs])"

Particularly when using Azure AD with identity pools, the ability to define values for the extensions "logicalCluster" and "identityPoolId" is mandatory.