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

[Martin Storsjö via llvm-commits]

================
@@ -29,8 +31,48 @@ 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. However, there are
+several common alternatives.
+
+To illustrate, let's assume you're working on two branches in your fork of the
+``llvm/llvm-project`` repository, and you want to eventually merge both into ``main``:
+
+* `feature_1`, which contains commit `feature_commit_1`,
+* `feature_2`, which contains commit `feature_commit_2` and depends on
+  `feature_1` (so it also includes `feature_commit_1`).
+
+Your options are as follows:
+
+#. Two PRs with dependency note
+
+  Create PR_1 for branch feature_1 and PR_2 for branch feature_2. In PR_2, add a
+  note in the PR summary indicating that it is part of a series or depends on
+  another PR (e.g., “Depends on #PR_1”). It's also helpful to highlight which
+  commits belong to the base PR, so reviewers can focus on the new changes.
+
+#. User branches in `llvm/llvm-project`
+
+  Create user branches in the main repository, as described
+  :ref:`above<github_branches>`. Then:
+
+  * Create a pull request from `users/<username>/feature_1` → `main`
+  * Create another from `users/<username>/feature_2` →
+    `users/<username>/feature_1` Once `feature_1` is merged, you can update the
+    base of the second PR to target main.
+
+#. Use a stacked PR tool
+
+  Use a tool like SPR or Graphite (described below) to manage stacked PRs more
+  easily.
+
+Each of these approaches can help streamline the review process. Choose the one
----------------
mstorsjo wrote:

> Sure. I just need to make sure I am not duplicating info. "squash and merge" is already covered here:
> 
> https://llvm.org/docs/GitHub.html#landing-your-change

Right, I see - I haven't checked the full context (and I did indeed think your reply was to Simon).

IMO, even if it duplicates info a little bit, it may still be good to at least briefly mention that this is due to us using "squash and merge".

Also now from looking at the full document, it feels like the order of info is a bit weird, when it starts out with how to create branches in the repo and using graphite, and only then giving the high level info about using PRs for contributions. But that's of course a separate discussion.

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