[llvm] [llvm][docs] Document how to get admin permissions for a Buildbot worker (PR #108561)

[Alex Bradbury via llvm-commits]

asb wrote:

> The key point here is that Buildbot sees the email listed on your public GitHub profile page. That email must therefore be associated with the account, but it can be primary or secondary, as long as you've chosen to show it on your profile.

You know I think this is the key reason for my confusion, because when you login and grant oauth permissions to the buildbot it requests permission to "Access user email addresses (read-only)". ie. the user:email permission [here](https://docs.github.com/en/apps/oauth-apps/building-oauth-apps/scopes-for-oauth-apps) which should allow listing all email addresses https://docs.github.com/en/rest/users/emails?apiVersion=2022-11-28#list-email-addresses-for-the-authenticated-user

https://github.com/llvm/llvm-project/pull/108561