[NFC] Add more information on testing MDVals insertion in EmitDxilFunctionProps (#5335)

A recent change from a recent PR includes changes to how the MDVals data
structure inside of EmitDxilFunctionProps gets elements (push_back
instead of index assignment).
However, there was no accompanying test for these changes, because
HLSLFileCheck\hlsl\workgraph\called_function_arg_nodeoutput.hlsl
exercised this code path and failed without the MDVals changes.
This PR adds some extra context to the above test so that the extra
purpose the above test serves isn't lost with time.

The test passes with the MDVals pushback changes, and fails with the
original valIdx index assignments.

tools/clang/test/HLSLFileCheck/hlsl/workgraph/called_function_arg_nodeoutput.hlsl

@@ -10,6 +10,13 @@
 // outputNode.GetGroupNodeOutputRecords(13):
 // CHECK_FCGL: call %dx.types.NodeRecordHandle @"dx.hl.op..%dx.types.NodeRecordHandle (i32, %dx.types.NodeHandle, i32)"(i32 {{[0-9]+}}, %dx.types.NodeHandle %[[outputNodeHandle]], i32 13)
 
+// This test also exercises the path that fills in the maximum amount of entries into MDVals in
+// the EmitDxilFunctionProps function in DxilMetadataHelper.cpp. This specific path can only be 
+// exercised when node shaders are defined. Previously, the data structure would receive
+// too many entries, and be assigned out of bounds due to the method of index assignment.
+// Ever since the insertion method has been changed to push_back, the fact this test passes
+// proves that the switch from index assignment to push_back is fine. 
+
 struct loadStressRecord
 {
     uint3 grid : SV_DispatchGrid;