bar: add note to setGroupPositionsInOverlayMode

* Added note to explain the reason why setGroupPositionsInOverlayMode
  handles the case barnorm is defined.

Author: n-riesco

src/traces/bar/set_positions.js

@@ -124,6 +124,10 @@ function setGroupPositionsInOverlayMode(gd, pa, sa, calcTraces) {
         setOffsetAndWidth(gd, pa, sieve);
 
         // set bar bases and sizes, and update size axis
+        //
+        // (note that `setGroupPositionsInOverlayMode` handles the case barnorm
+        // is defined, because this function is also invoked for traces that
+        // can't be grouped or stacked)
         if(barnorm) {
             sieveBars(gd, sa, sieve);
             normalizeBars(gd, sa, sieve);