[docs] Update docs with new admin privilege

Epic #4192 

## Description

Given that the Admin privilege can do everything, update the permissions document (https://concourse-ci.org/user-roles.html) to reflect this new state of the admin privilege.

## PR
https://github.com/concourse/docs/pull/245