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

[David Spickett via llvm-commits]

DavidSpickett wrote:

And if you're wondering, the console output looks like:
```
{"jsonrpc":"2.0","id":332,"error":{"code":-32600,"message":" You must be the worker owner."}}
<as many times as there are pending requests>
```
I'm not sure why I thought to check it at all, so I figured it was worth mentioning even though it's a bit of an unintended feature.

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