[llvm] Update GettingStarted.rst doc with negative refspec to filter user branches (PR #75015)

[Danila Malyutin via llvm-commits]

danilaml wrote:

When I do both
```
  git config --add remote.origin.fetch '^refs/heads/users/*'
  git config --add remote.origin.fetch '^refs/heads/revert-*'
  ```
  Only the last one used is showing up on `git config remote.origin.fetch`. Was this the intended effect? Docs read as if the above commands would filter both the user and revert branches not one or the other.

https://github.com/llvm/llvm-project/pull/75015