Add comment explaining early guard

esphome/components/ld2420/ld2420.cpp

@@ -541,6 +541,8 @@ void LD2420Component::handle_simple_mode_(const uint8_t *inbuf, int len) {
 }
 
 void LD2420Component::read_batch_(std::span<uint8_t, MAX_LINE_LENGTH> buffer) {
+  // 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;