Add clarifying comment.

src/Compilers/VisualBasic/Test/IOperation/IOperation/IOperationTests_IOmittedArgumentOperation.vb

@@ -32,6 +32,10 @@ End Class
 
             Dim methodOperation = semanticModel.GetOperation(method)
 
+            ' When we fetch an omitted argument that involves a conversion, there was a bug that involved
+            ' a synthesized conversion node being introduced without a corresponding BoundNode that got
+            ' incorrectly duplicated. This excercises that code path to ensure that the BoundNode is correctly
+            ' cached in the tree.
             Dim operationsToAnalyze = methodOperation.DescendantsAndSelf().ToArray()
         End Sub