[llvm] [llvm][docs] Add information about using GitHub's UI for updating a branch (PR #166625)

[Aaron Ballman via llvm-commits]

================
@@ -82,18 +82,20 @@ Updating Pull Requests
 ----------------------
 In order to update your pull request, the only thing you need to do is to push
 your new commits to the branch in your fork. That will automatically update
-the pull request.
+the pull request. You can also use the Update Branch button in GitHub's Pull
+Request UI, but be aware that it will create a merge commit on your branch.
 
 When updating a pull request, you should push additional "fix up" commits to
 your branch instead of force pushing. This makes it easier for GitHub to
 track the context of previous review comments. Consider using the
 `built-in support for fixups <https://git-scm.com/docs/git-commit#Documentation/git-commit.txt---fixupamendrewordltcommitgt>`_
 in git.
 
-If you do this, you must squash and merge before landing the PR and
-you must use the pull request title and description as the commit message.
-You can do this manually with an interactive git rebase or with GitHub's
-built-in tool. See the section about landing your fix below.
+If you create fix up or merge commits, you must squash and merge before
----------------
AaronBallman wrote:

We still allow direct pushes to main, so maybe the CLI allows for pushing a non-squashed merge?

https://github.com/llvm/llvm-project/pull/166625