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

[Alex Bradbury via llvm-commits]

asb wrote:

Firstly, thank you for documenting this - I recently had to reach out to Galina for help with this, but I failed to think to follow up to document it.

I know it's post-commit, but two thoughts:
* Might it be better to suggest people go to to https://github.com/settings/emails while logged in to review the emails associated with their account? For me at least, github.com/asb doesn't show anything very useful.
* I was surprised to hear it's working for you with an email other than your primary one. I wasn't able to get login to work on the lab.llvm.org until I switched the admin email for my bots over to asb at asbradbury.org (my GH primary email) rather than asb at igalia.com (the backup email). It's possible there was something else going on (e.g. caching) that caused me to think that was the fix.

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