Update change note for compound-assignment coverage

Author: Copilot

change_notes/2026-05-18-fix-fp-misra-c-13-4.md

@@ -1,4 +1,4 @@
 - `RULE-13-4` - `ResultOfAnAssignmentOperatorShouldNotBeUsed`:
-  - Fixed false positives.
+  - Fixed false positives and false negatives by reporting compound assignments (for example, `+=`).
 - `RULE-8-18-2` - `ResultOfAnAssignmentOperatorShouldNotBeUsed`:
-  - Fixed false positives.
+  - Fixed false positives and false negatives by reporting compound assignments (for example, `+=`).