Consolidate comments in plan_2_interest_rate

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: MaxGhenis

policyengine_uk/variables/gov/hmrc/student_loans/plan_2_interest_rate.py

@@ -21,13 +21,11 @@ def formula(person, period, parameters):
         rpi = parameters(period).gov.economic_assumptions.yoy_growth.obr.rpi
 
         # Per Regulation 21AB, lower interest threshold = repayment threshold
-        # Below lower threshold: RPI only
-        # Above upper threshold: RPI + 3%
-        # Between: linear taper
+        # Below lower: RPI only; above upper: RPI + 3%; between: linear taper
         taper_fraction = np.clip(
             (income - p.thresholds.plan_2)
             / (p.interest_rates.plan_2.upper_threshold - p.thresholds.plan_2),
             0,
             1,
         )
-        return rpi + (p.interest_rates.plan_2.additional_rate * taper_fraction)
+        return rpi + p.interest_rates.plan_2.additional_rate * taper_fraction