Add comment explaining early guard

esphome/components/rd03d/rd03d.cpp

@@ -81,6 +81,8 @@ void RD03DComponent::dump_config() {
 }
 
 void RD03DComponent::loop() {
+  // Early return avoids stack adjustment for the batch buffer below.
+  // loop() runs ~7000/min so most calls have nothing to read.
   int avail = this->available();
   if (avail <= 0)
     return;