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

[David Spickett via llvm-commits]

================
@@ -29,8 +31,65 @@ 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
+=====================
+
+To separate related changes or to break down a larger PR into smaller, reviewable
+pieces, use "stacked pull requests" — this helps make the review process
+smoother. While GitHub does not natively support stacked pull requests, there
+are several common alternatives.
+
+To illustrate, assume that you are 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 a dependency note
+
+   Create PR_1 for `feature_1` and PR_2 for `feature_2`. In PR_2, include a
+   note in the PR summary indicating that it depends on PR_1 (e.g.,
+   “Depends on #PR_1”).
+
+   To make review easier, clearly highlight which commits are part of the base
+   PR and which are new. This helps reviewers focus only on the incremental
+   changes.
+
+#. Use user branches in ``llvm/llvm-project``
+
+   Create user branches in the main repository, as described
+   :ref:`above<github_branches>`. Then:
+
+   - Open a pull request from `users/<username>/feature_1` → `main`
+   - Open another from `users/<username>/feature_2` → `users/<username>/feature_1`
+
+   This approach allows GitHub to display clean, incremental diffs for each PR
+   in the stack, making it much easier for reviewers to see what’s changed
+   at each step. Once `feature_1` is merged, you can rebase and re-target
+   `feature_2` to `main`.
+
+#. Use a stacked PR tool
+
+   Use tools like SPR or Graphite (described below) to automate managing
+   stacked PRs. These tools are also based on using user branches
+   in ``llvm/llvm-project``.
+
+.. note::
+   Without using user branches, GitHub will not display a proper diff for
+   subsequent PRs in the stack. Instead, it will show a combined diff that
+   includes all commits in the stack. It is the PR author’s responsibility to
+   make it clear which commits are relevant to the current PR.
----------------
DavidSpickett wrote:

Given that "make it clear" is essentially what one has to do in "two PRs with a dependency note", consider:
* Using the same verb in each section, so it's easier to realise oh right I was already told how to do that. Currently one is "make clear" and the other is "highlight".
* Adding something like "as if you had used method 1" or "as you would if you had not used a stacked PR tool", so readers know to look back for guidance.

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