docs: clarify ESP32 mempool version gate

Author: Copilot

src/nimble/porting/nimble/include/os/os_mempool.h

@@ -37,6 +37,7 @@
 #endif
 
 #if defined(ESP_ARDUINO_VERSION)
+/* Arduino ESP32 core 3.3.11+ dropped the legacy r_ prefix for these mempool symbols. */
 #define NIMBLE_OS_MEMPOOL_USE_ROM_R_PREFIX (ESP_ARDUINO_VERSION < ESP_ARDUINO_VERSION_VAL(3, 3, 11))
 #else
 #define NIMBLE_OS_MEMPOOL_USE_ROM_R_PREFIX 1
@@ -415,6 +416,7 @@ os_error_t os_memblock_put(struct os_mempool *mp, void *block_addr);
 }
 #endif
 
+/* Keep this version gate local to this header. */
 #undef NIMBLE_OS_MEMPOOL_USE_ROM_R_PREFIX
 
 #endif  /* _OS_MEMPOOL_H_ */