contributing: clarify when to update a branch vs avoid during review

[ali90h]

Explain when to update (merge conflicts, upstream CI breakage, maintainer request) and when not to (already-green PRs under review).

Recommend incremental commits during review; prefer squash/rebase at the end when requested.

Mention git push --force-with-lease, a brief “what changed” note, and running local checks.

Addresses #2542

Test plan: mdbook build succeeds locally; visual check confirms the new guidance replaces the old prescriptive text.

[rustbot]

Thanks for the PR. If you have write access, feel free to merge this PR if it does not need reviews. You can request a review using r? rustc-dev-guide or r? <username>.

[ali90h]

r? rustc-dev-guide

[jyn514]

thanks! I agree this is a good change. I have a few nits about wording if it's not too much of a hassle.

[ali90h]

thanks! I agree this is a good change. I have a few nits about wording if it's not too much of a hassle.

Thanks for your feedback🙏, it has been modified.