Add explanation to remaining std::min calls

src/CanvasRenderingContext2d.cc

@@ -619,6 +619,8 @@ NAN_METHOD(Context2d::PutImageData) {
       if (dx < 0) sw += dx, sx -= dx, dx = 0;
       if (dy < 0) sh += dy, sy -= dy, dy = 0;
       // clamp width at canvas size
+      // Need to wrap std::min calls using parens to prevent macro expansion on
+      // windows. See http://stackoverflow.com/questions/5004858/stdmin-gives-error
       cols = (std::min)(sw, context->canvas()->width - dx);
       rows = (std::min)(sh, context->canvas()->height - dy);
       break;