docs: add warning for doxygen version

Running with a newer doxygen version than what's
used in CI may raise warnings locally that are not
present when built by CI


* Original commit: espressif/esp-idf@9b552ce004

components/freemodbus/common/include/esp_modbus_master.h

@@ -136,7 +136,6 @@ esp_err_t mbc_master_init(mb_port_type_t port_type, void** handler);
  * @brief Initialize Modbus Master controller interface handle
  *
  * @param[in] handler - pointer to master data structure
- * @return None
  */
 void mbc_master_init_iface(void* handler);
 

components/freemodbus/common/include/esp_modbus_slave.h

@@ -78,7 +78,6 @@ esp_err_t mbc_slave_init(mb_port_type_t port_type, void** handler);
  * @brief Initialize Modbus Slave controller interface handle
  *
  * @param[in] handler - pointer to slave interface data structure
- * @return None
  */
 void mbc_slave_init_iface(void* handler);