`UIPermission` class has no implementation but docs don't mention it

[`UIPermission`](https://docs.microsoft.com/dotnet/api/system.security.permissions.uipermission) type has an extensive documentation associated with it, however looking through the [code](https://github.com/dotnet/runtime/blob/master/src/libraries/System.Security.Permissions/src/System/Security/Permissions/UIPermission.cs) has no implementation.

This is extremely misleading and confusing. I've heard anecdotes that CAS is not longer used, hence this type has implementation, but the docs do no make this bleeding obvious.

Applies to all other related types.