layers:Add TODO comments for valid usage updates

Note a few unique error enum duplicates that should have their own
unique ids added through spec updates.
diff --git a/layers/descriptor_sets.cpp b/layers/descriptor_sets.cpp
index 902a71c..6de07cc 100644
--- a/layers/descriptor_sets.cpp
+++ b/layers/descriptor_sets.cpp
@@ -1221,6 +1221,7 @@
     // Verify consecutive bindings match (if needed)
     if (!p_layout_->VerifyUpdateConsistency(update->dstBinding, update->dstArrayElement, update->descriptorCount, "write update to",
                                             set_, error_msg)) {
+        // TODO : Should break out "consecutive binding updates" language into valid usage statements
         *error_code = VALIDATION_ERROR_00938;
         return false;
     }