Improve logout message

[SteelWagstaff]

We should consider providing a more descriptive message for the user once they've logged out of Pressbooks after using SSO to let them know that they've been logged out of SSO but may need to close their browser entirely to complete the logout.

A client writes

Usually closing a browser is required to complete a log out session completely, as sometimes if you don't close the close the browser, refreshing or going back to the application in the same session will re-log a user in. Perhaps if a message such as the one you have when you log out of the library is applied on log out (which advises to close the browser to complete the log out) would help.