[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
+alternatives:
+
+  * Add a note in your PR summary indicating that your patch is part of a
+    series or depends on another PR (e.g., “Depends on #123456”). It also helps
+    to highlight which commits belong to other PRs, so reviewers can focus only
----------------
boomanaiden154 wrote:

Is anyone actually using the workflow where multiple PRs are opened where PR x has its commit and all the commits it depends on? User branches should be used for that and it makes the review much easier.

When using SPR though, it is good to add a note summarizing everything.

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