adds explanation comment

lib/torque/leaflet/torque.js

@@ -35,6 +35,7 @@ L.TorqueLayer = L.CanvasLayer.extend({
     });
     this.prevRenderedKey = 0;
     if (options.cartocss) {
+      // We're only passing the Map header to the global options because the parser won't like turbocarto expressions
       var headerCartoCSS = options.cartocss.replace(/\n/g,'').match(/Map\s*?\{.*?}/g)[0];
       torque.extend(options, torque.common.TorqueLayer.optionsFromCartoCSS(headerCartoCSS));
     }