Add comment (false positive "sonarcloud" report).

commons-math-legacy/src/main/java/org/apache/commons/math4/legacy/analysis/polynomials/PolynomialsUtils.java

@@ -366,7 +366,9 @@ public final class PolynomialsUtils {
     private static PolynomialFunction buildPolynomial(final int degree,
                                                       final List<BigFraction> coefficients,
                                                       final RecurrenceCoefficientsGenerator generator) {
-
+        // Synchronizing on a method parameter is not safe; however, in this
+        // case, the lock object is an immutable field that belongs to this
+        // class.
         synchronized (coefficients) {
             final int maxDegree = (int) AccurateMath.floor(AccurateMath.sqrt(2 * coefficients.size())) - 1;
             if (degree > maxDegree) {