add comment explaining increased layer thickness

Tracking/Acts/FaserActsGeometry/src/FaserActsLayerBuilder.cxx

@@ -206,6 +206,7 @@ FaserActsLayerBuilder::buildLayers(const Acts::GeometryContext& gctx,
   std::shared_ptr<const Acts::ProtoSurfaceMaterial> materialProxy = nullptr;
 
   double layerZ = 0.5 * (pl.min(Acts::binZ) + pl.max(Acts::binZ));
+  // increase layerThickness by 4 mm so that after alignment shifts all modules are still within the boundaries
   double layerThickness = (pl.max(Acts::binZ) - pl.min(Acts::binZ)) + 4_mm;
   double layerZInner = layerZ - layerThickness/2.;
   double layerZOuter = layerZ + layerThickness/2.;