Simplify wording on the code review docs

What does this MR do?

The current wording on the MR review guidelines is problematic:

The merge request author resolves only the threads they have fully addressed. If there's an open reply, an open thread, a suggestion, a question, or anything else, the thread should be left to be resolved by the reviewer.

This leaves very few cases where the author would be able to resolve a thread. It's probably a lot safer and simpler to just say the following:

Open threads should only be resolved by the reviewer.

My arguments for this are the following:

1. Resolved diffs are hard to see so it's tricky to see which points were addressed in that review.
2. "fully addressed" is really the reviewer's call.
3. The more complicated we make the wording, the more likely it is to be mis-interpreted.

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• [-] If applicable, update the permissions table.
• [-] Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the documentation label.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.