Clarify Renovate merging instructions (#1641)

# Pull Request

## 🤨 Rationale


[Discussion](#1634 (comment))
about the best way to update a Renovate PR.

## 👩‍💻 Implementation

Add note to CONTRIBUTING

## 🧪 Testing

N/A

## ✅ Checklist

<!--- Review the list and put an x in the boxes that apply or ~~strike
through~~ around items that don't (along with an explanation). -->

- [ ] I have updated the project documentation to reflect my changes or
determined no changes are needed.

CONTRIBUTING.md

@@ -181,7 +181,7 @@ npm install @ni/nimble-tokens --workspace=@ni/nimble-components
 
 This repository uses [Renovate](https://docs.renovatebot.com/) to automatically create pull requests that bump the version of dependencies on a schedule. Renovate is configured via [`renovate.json`](./.github/renovate.json).
 
-Code owners are responsible for completing or rejecting Renovate PRs. Completing a PR may require manually adding a beachball change file to the branch. The change `type` will typically be `patch` if any `package.json` is changing. The `comment` should summarize which set of dependencies are being updated. To complete a PR you may need to manually trigger a rebase by clicking the checkbox in the PR description.
+Code owners are responsible for completing or rejecting Renovate PRs. Completing a PR may require manually adding a beachball change file to the branch. The change `type` will typically be `patch` if any `package.json` is changing. The `comment` should summarize which set of dependencies are being updated. To complete a PR you may need to manually trigger a rebase by clicking the checkbox in the PR description. **Note:** prefer the checkbox over GitHub's "Update branch" button so that Renovate can remain in control of all commits to its branch.
 
 ## Handling intermittent test failures