Update Google docs about refresh tokens support

[matheuscscp]

Hi! Thanks very much for Dex!

Refresh tokens were working great until I set promptType to "". I see these logs on Dex when trying to use a refresh token with this setting:

time=2025-12-23T11:27:45.787Z level=ERROR msg="failed to refresh identity" err="google: failed to get token: oauth2: token expired and refresh token is not set" request_id=dcb2ecf2-720f-42a1-ae54-4feb7e1fd868
time=2025-12-23T11:27:45.797Z level=ERROR msg="failed to update refresh token" err="refresh token error: status 500, \"invalid_request\" " request_id=dcb2ecf2-720f-42a1-ae54-4feb7e1fd868