Document OIDC_ENABLED in settings.rst

[charleswhchan]

Fixes #

Description of the Change

Checklist

• [ ] PR only contains one change (considered splitting up PR)
• [ ] unit-test added
• [x] documentation updated
• [ ] CHANGELOG.md updated (only for user relevant changes)
• [ ] author name in AUTHORS

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.56%. Comparing base (560f84d) to head (4d20a2e). Report is 7 commits behind head on master.

:exclamation: Current head 4d20a2e differs from pull request most recent head 0d5fd0f. Consider uploading reports for the commit 0d5fd0f to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1408 +/- ## ======================================= Coverage 97.56% 97.56% ======================================= Files 32 32 Lines 2132 2132 ======================================= Hits 2080 2080 Misses 52 52 ```

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