math/cmplx: added clarifying comment

Fixes #14890.

Change-Id: Ie790276b0e2ef94c92db3a777042d750269f876a
Reviewed-on: https://go-review.googlesource.com/20953
Reviewed-by: Alan Donovan <adonovan@google.com>

src/math/cmplx/cmath_test.go

@@ -9,6 +9,9 @@ import (
 	"testing"
 )
 
+// The higher-precision values in vc26 were used to derive the
+// input arguments vc (see also comment below). For reference
+// only (do not delete).
 var vc26 = []complex128{
 	(4.97901192488367350108546816 + 7.73887247457810456552351752i),
 	(7.73887247457810456552351752 - 0.27688005719200159404635997i),