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

GitHub's Update Branch button is a helpful tool for quickly updating a
PR before merging, but it might also be important to point out that it
creates a merge commit without additional prompting, which may or may
not be desired behavior for a given LLVM contributor.

Opened on the suggestion of @lamb-j

Author: MixedMatched

llvm/docs/GitHub.rst

@@ -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
+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.
 
 When pushing to your branch, make sure you push to the correct fork. Check your
 remotes with:
@@ -108,7 +110,9 @@ Rebasing Pull Requests and Force Pushes
 ---------------------------------------
 In general, you should avoid rebasing a Pull Request and force pushing to the
 branch that's the root of the Pull Request during the review. This action will
-make the context of the old changes and comments harder to find and read.
+make the context of the old changes and comments harder to find and read. If
+you want to make your pull request up-to-date with main, you might consider
+updating your branch, as described in the previous section.
 
 Sometimes, a rebase might be needed to update your branch with a fix for a test
 or in some dependent code.