Updated whatsnew entry

Author: mhaseebtariq

doc/source/whatsnew/v0.19.0.txt

@@ -361,7 +361,7 @@ Google BigQuery Enhancements
 Other enhancements
 ^^^^^^^^^^^^^^^^^^
 
-- The ``.get_credentials()`` method of ``GbqConnector`` can now first try to fetch [the application default credentials](https://developers.google.com/identity/protocols/application-default-credentials). This is only valid if the parameter ``private_key`` is not provided. Using this functionality, the ``OAuth2WebServerFlow`` call can be skipped (:issue:`13577`).
+- The ``.get_credentials()`` method of ``GbqConnector`` can now first try to fetch [the application default credentials](https://developers.google.com/identity/protocols/application-default-credentials). See the :ref:`docs <io.bigquery_authentication>` for more details (:issue:`13577`).
 
 - The ``.tz_localize()`` method of ``DatetimeIndex`` and ``Timestamp`` has gained the ``errors`` keyword, so you can potentially coerce nonexistent timestamps to ``NaT``. The default behaviour remains to raising a ``NonExistentTimeError`` (:issue:`13057`)
 - ``pd.to_numeric()`` now accepts a ``downcast`` parameter, which will downcast the data if possible to smallest specified numerical dtype (:issue:`13352`)