Add comment about sensing block id oddness

src/blocks/scratch3_sensing.js

@@ -84,6 +84,9 @@ class Scratch3SensingBlocks {
                 getId: () => 'timer'
             },
             sensing_current: {
+                // This is different from the default toolbox xml id in order to support
+                // importing multiple monitors from the same opcode from sb2 files,
+                // something that is not currently supported in scratch 3.
                 getId: (_, param) => `current_${param}`
             }
         };