[UX] Append `This role receives all new permissions by default.` text to the description of the admin role

Description of the bug

Can we please consider appending the This role receives all new permissions by default. text to the description of whichever role has been selected to be the admin role? (according to the user_admin_role setting in system.core.json) ...I feel that that would still provide value, and it won't be a lie if that's what has been configured on the site.

This is a follow-up to https://github.com/backdrop/backdrop-issues/issues/5748