Document entry-year contribution proration

src/ccma/rules/scripts/contribution_claims.py

@@ -82,6 +82,8 @@ def _membership_claims(context: RuleContext, started_at: date, accepted_at: date
     actions = []
     monthly_amount = annual_amount / Decimal(12)
     for suffix, first_month, last_month, regular_due in periods:
+        # The entry year is intentionally billed from the entry month onward,
+        # even when retroactive claims create old membership-fee claims.
         charged_from = max(first_month, period_start.month)
         months = max(0, last_month - charged_from + 1)
         if months == 0: