Include OIDC_CLIENT_SECRETS in documentation

[Alwinius]

The configuration variable OIDC_CLIENT_SECRETS is not documented here: https://flask-oidc.readthedocs.io/en/latest/ but a very important one for using the framework.

[svintit]

If you want this resolved i'm maintaining a fork of this with some added extensibility. Feel free to open a PR/issue here:

• https://github.com/svintit/flask-oidc-ext/pulls