[llvm] [Docs] Update contribution guide to remove the single-commit requirement (PR #212031)

[via llvm-commits]

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@@ -51,7 +57,18 @@ Once you have a patch ready, it is time to submit it. The patch should:
 * conform to the {doc}`CodingStandards`. You can use the [clang-format-diff.py] or [git-clang-format] tools to automatically format your patch properly.
 * not contain any unrelated changes
 * be an isolated change. Independent changes should be submitted as separate patches as this makes reviewing easier.
-* have a single commit, up-to-date with the upstream `origin/main` branch, and don't have merges.
+
+```{note}
+When a PR is accepted and submitted into mainline, all changes from the PR are combined into a single commit using GitHub's "Squash and Merge" button. (We don't use merge commits on LLVM's mainline.) Because of that policy, the series of commits inside a PR branch does not make it into the "permanent record", and is thus not of critical importance.
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Sirraide wrote:

Ok, I believe that page also needs updating because it says

> If your PR contains multiple commits, you need to consolidate those commits into one commit. There are three different ways to do this, [...] Interactive rebase with fixups. This is the recommended method

and I at least would argue that the 'recommended method' should be to just press Squash+Merge (which is also listed as an option there). Are we fine w/ me updating that document too as part of this?

https://github.com/llvm/llvm-project/pull/212031