Add a comment about using mpmath to generated the reference values.

Author: WarrenWeckesser

tests/xsf_tests/test_cyl_bessel_k.cpp

@@ -6,6 +6,7 @@
 TEST_CASE("cyl_bessel_k gh-46", "[cyl_bessel_k][xsf_tests]") {
     using test_case = std::tuple<double, std::complex<double>, std::complex<double>, double>;
     using std::complex;
+    // Reference values were computed with the Python library mpmath.
     auto [v, z, ref, rtol] = GENERATE(
         test_case{0.0, complex{680.0, -1000.0}, complex{1.901684871999608e-298, 1.713412341479591e-297}, 1e-13},
         test_case{0.0, complex{680.0, -680.0}, complex{-4.553730032944803e-298, 1.878727010109855e-297}, 1e-13},