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

[David Spickett via llvm-commits]

DavidSpickett 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.

I see you have `asb at asbradbury.org` on your public profile, so by switching to that now Buildbot sees it on your public profile as well and you're seen as the admin.

This is what had me stuck for ages, it was only by chance a colleague interpreted "GitHub email" to mean "GitHub profile email" and we realised what was happening.

That said, if that's not clear I'll update the wording.

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