Add comment explaining all-at-once

src/blocks/scratch3_control.js

@@ -185,6 +185,12 @@ class Scratch3ControlBlocks {
     }
 
     allAtOnce (args, util) {
+        // Since the "all at once" block is implemented for compatiblity with
+        // Scratch 2.0 projects, it behaves the same way it did in 2.0, which
+        // is to simply run the contained script (like "if 1 = 1").
+        // (In early versions of Scratch 2.0, it would work the same way as
+        // "run without screen refresh" custom blocks do now, but this was
+        // removed before the release of 2.0.)
         util.startBranch(1, false);
     }
 }