Add comment

core/blockly.js

@@ -202,6 +202,9 @@ Blockly.onKeyDown_ = function(e) {
     }
     if (Blockly.selected &&
         Blockly.selected.isDeletable() && Blockly.selected.isMovable()) {
+      // Don't allow copying immovable or undeletable blocks. The next step
+      // would be to paste, which would create additional undeletable/immovable
+      // blocks on the workspace.
       if (e.keyCode == 67) {
         // 'c' for copy.
         Blockly.hideChaff();