[llvm] [docs][GitHub] Document alternative approach to stacked PRs (PR #132424)

[Aiden Grossman via llvm-commits]

================
@@ -29,8 +29,23 @@ intended to be able to support "stacked" pull-request. Do not create any branche
 llvm/llvm-project repository otherwise, please use a fork (see below). User branches that
 aren't associated with a pull-request **will be deleted**.
 
+Stacked Pull Requests
+=====================
+
+GitHub does not natively support stacked pull requests. There are two common
----------------
boomanaiden154 wrote:

Yes, user branches will give you stacked PRs. You can do them manually, but SPR will set them up for you automatically.

User branches aren't great, but I wouldn't say they're discouraged. The official (maybe just de facto?) policy is that they are perfectly fine to be used for stacked PRs, but should be avoided for PRs that are not stacked.

https://github.com/llvm/llvm-project/pull/132424