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

[Vitaly Buka via llvm-commits]

https://github.com/vitalybuka approved this pull request.

I assume we going to have SRP setup somewhere.

Looks like everyone I asked to try SPR failed to realize
that branch destination is main repo and branchPrefix must be specific.


```
[spr]
	githubRemoteName = origin
	githubRepository = llvm/llvm-project
	githubMasterBranch = main
	branchPrefix = users/vitalybuka/spr/
	requireTestPlan = false
```

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