Missing DocBlocks

Though it's likely low-priority, it would be great if all classes and functions were documented with DocBlocks to follow WordPress' [PHP Documentation Standards](https://developer.wordpress.org/coding-standards/inline-documentation-standards/php/).

For example, most of the functions and classes in https://github.com/Automattic/wp-openid-connect-server/tree/main/src don't contain documentation.

Thank you for considering!

Review the PHP files under src and the linked WordPress PHP Documentation Standards first. Identify classes and functions that lack DocBlocks, then add documentation consistent with those standards across the source tree. Done means the relevant classes and functions in src have DocBlocks.

Written by the indexing model from the issue text.