[llvm] Update GitHub doc to mention that we accepts user branches for Stacked PRs (PR #73774)

[Tom Stellard via llvm-commits]

https://github.com/tstellar commented:

Is there a local config we can use to tell git not to pull these branches?  If so, it might be good to document that.

https://github.com/llvm/llvm-project/pull/73774