[ethernet] Document lock-free polling rationale in RP2040 loop

Explain why linkStatus() and connected() are called without
LwIPLock — linkStatus() only reads the W5500 PHY via SPI, and
connected() does a single atomic 32-bit read of netif->ip_addr.

esphome/components/ethernet/ethernet_component_rp2040.cpp

@@ -80,7 +80,11 @@ void EthernetComponent::setup() {
 }
 
 void EthernetComponent::loop() {
-  // On RP2040, we need to poll connection state since there are no events
+  // On RP2040, we need to poll connection state since there are no events.
+  // linkStatus() reads the W5500 PHY register via SPI — no lwip state involved.
+  // connected() reads netif->ip_addr without LwIPLock, but this is a single
+  // 32-bit aligned read (atomic on ARM) — worst case is a one-iteration-stale
+  // value, which is benign for polling.
   if (this->eth_ != nullptr) {
     bool link_up = this->eth_->linkStatus() == LinkON;
     bool has_ip = this->eth_->connected();