docs updated for what "default" branch protection means for global setting.

What does this MR do?

Improves docs for global setting of Admin area > Settings > General > Visibility and access control for "default branch protection".

We had some questions around this global setting for "default branch protection" and differences from per repository settings and reached out to Support for clarity. Diana Stanley (Gitlab Senior Support Engineer) replied via email (Thanks Diana!!!) and we decided to contribute back to the docs to help other companies & teams.

Related issues

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.