Improve RBAC security documentation

# Improve documentation ## Link [Custom Claims & Role-based Access Control (RBAC)](https://supabase.com/docs/guides/database/postgres/custom-claims-and-role-based-access-control-rbac) ## Describe the problem The documentation is mostly helpful, however it leaves off a critical step of applying RLS to the newly created `user_role` and `user_permissions` table. There lines of SQL present in the slack clone migration files, but this should be present in the documentation to prevent footguns related to particularly sensitive portions of an application. Yes, if you're implementing RBAC you probably should have a good handle on RLS by now and it should be intuitive to you, but I think it's more caring to your users to provide those details up front and help them prevent auth related security issues. ## Describe the improvement Docs should include details for enabling RLS on newly created RBAC tables, as well as setting appropriate RLS policies to secure them. It may also be worth revisiting best practices here to verify the slack clone approach is a broadly recommended approach to securing these new RBAC tables. ## Additional context Pertinent migrations in the slack clone: - https://github.com/supabase/supabase/blob/master/examples/slack-clone/nextjs-slack-clone/supabase/migrations/20240214102356_init.sql#L78-L79 - https://github.com/supabase/supabase/blob/master/examples/slack-clone/nextjs-slack-clone/supabase/migrations/20240214102356_init.sql#L92