Update documentation for the 3.x branch

Problem/Motivation

There are several changes to the module settings. We should update the on-line documentation at https://www.drupal.org/docs/8/modules/openid-connect/configuration (and maybe other pages in that guide) no later than the release of 3.0.0.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes