docs: clarify partial scoring logic in Branch-Protection tiered requirements

**Problem Description** The current documentation implies an all or nothing approach to tiers: "This test has tiered scoring. Each tier must be fully satisfied to achieve points at the next tier. For example, if you fulfill the Tier 3 checks but do not fulfill all the Tier 2 checks, you will not receive any points for Tier 3." This leads users to believe that if they miss one requirement in Tier 2, they will only receive the 3 points from Tier 1. However, the implementation in checks/evaluation/branch_protection.go uses normalizeScore to award partial credit for each passed requirement within the current tier. **Proposed Change** Update the wording in docs/checks/internal/checks.yaml to accurately reflect that while you can't jump to a higher tier without finishing the current one, you do get points for your progress within the tier. Suggested wording: "This test has tiered scoring. You receive partial credit for requirements met within your current tier. However, a tier must be fully satisfied before you can earn points from the next tier. For example, if you fulfill the Tier 3 checks but do not fulfill all the Tier 2 checks, you will not receive any points for Tier 3."