Add warning comment for --inst-debug-printf

This option is only for debugging/fixing debug printf instrumentation.
Users who wish to utilize debug printf should see the white paper at
vulkan.lunarg.com/doc.

Fixes #4908

source/opt/optimizer.cpp

@@ -445,6 +445,10 @@ bool Optimizer::RegisterPassFromFlag(const std::string& flag) {
   } else if (pass_name == "relax-float-ops") {
     RegisterPass(CreateRelaxFloatOpsPass());
   } else if (pass_name == "inst-debug-printf") {
+    // This option is not for user consumption. It is here to assist in
+    // debugging and fixing the debug printf instrumentation pass. For users who
+    // wish to utilize debug printf, see the white paper at
+    // vulkan.lunarg.com/doc
     RegisterPass(CreateInstDebugPrintfPass(7, 23));
   } else if (pass_name == "simplify-instructions") {
     RegisterPass(CreateSimplificationPass());