class oauth2.credentials.Credentials: API documentation is not exact about immutability

[eprigorodov]

The class description contains sentence "The credentials are considered immutable", which is not technically correct: refresh() method updates in place access token and expiration time.

Possible formulation: "The credentials are generally considered immutable, with the exception of the 'refresh()' method"

[eprigorodov]

Thank you for fixing and for the great product.