Connectors: Improve inline documentation for the Connectors API

Adds comprehensive PHPDoc to `connectors.php` and `class-wp-connector-registry.php` covering API overview, AI provider auto-discovery, admin UI integration, initialization lifecycle, authentication, usage examples, cross-references, and metadata override patterns.

Developed in https://github.com/WordPress/wordpress-develop/pull/11244.

Follow-up to [61943].

See #64791.




git-svn-id: https://develop.svn.wordpress.org/trunk@62032 602fd350-edb4-49c9-b593-d223f7449a82