uip.h: clarify comment
#define UIP_IP_PAYLOAD(ext) ((unsigned char *)UIP_IP_BUF_CHAR + UIP_IPH_LEN + (ext))
/**
- * Direct access to ICMP, UDP, and TCP headers and payload, with implicit ext header offset
+ * Direct access to ICMP, UDP, and TCP headers and payload, with implicit ext header offset (global uip_ext_len)
*/
#define UIP_ICMP_BUF ((struct uip_icmp_hdr *)UIP_IP_PAYLOAD(uip_ext_len))
#define UIP_ICMP_PAYLOAD ((unsigned char *)UIP_IP_PAYLOAD(uip_ext_len) + UIP_ICMPH_LEN)
