Add comment explaining available() <= 0 check

esphome/components/rd03d/rd03d.cpp

@@ -81,6 +81,8 @@ void RD03DComponent::dump_config() {
 }
 
 void RD03DComponent::loop() {
+  // All current UART available() implementations return >= 0,
+  // use <= 0 to future-proof against any that may return negative on error.
   int avail = this->available();
   if (avail <= 0)
     return;