Add documentation for group-level SPP REST API endpoint

Resolve "Add REST API endpoint to enable SPP fo... (!174342 - merged) • Miki Amos • 17.7 was merged and added a new REST API endpoint for group-level SPP modifications.

As of this comment, we should add documentation for this endpoint, highlighting how it doesn't change a setting that exists on the group-level but rather calls a background worker that goes over projects in the group and updates pre_receive_secret_detection_enabled setting for.

The documentation should be similar to the Project Security Settings API.