authz/github: do not recommend groupsCacheTTL, document allowGroupsPermissionsSync

Created by: bobheadxi

Adding example value makes groupsCacheTTL a recommended default, which we no longer want to push for. This PR makes the experimental status and niche use case more apparent, and removes the example.

Also documents the allowGroupsPermissionsSync introduced in https://github.com/sourcegraph/sourcegraph/pull/24328