Make lwIP compile with clang -Wdocumentation -> several documentation fixes

src/netif/ethernet.c

@@ -69,9 +69,9 @@ const struct eth_addr ethzero = {{0,0,0,0,0,0}};
  * @param p the received packet, p->payload pointing to the ethernet header
  * @param netif the network interface on which the packet was received
  * 
- * @see @ref LWIP_HOOK_UNKNOWN_ETH_PROTOCOL,
- * @ref ETHARP_SUPPORT_VLAN and
- * @ref LWIP_HOOK_VLAN_CHECK.
+ * @see LWIP_HOOK_UNKNOWN_ETH_PROTOCOL
+ * @see ETHARP_SUPPORT_VLAN
+ * @see LWIP_HOOK_VLAN_CHECK
  */
 err_t
 ethernet_input(struct pbuf *p, struct netif *netif)
@@ -247,7 +247,7 @@ free_and_return:
  * Send an ethernet packet on the network using netif->linkoutput().
  * The ethernet header is filled in before sending.
  *
- * @see @ref LWIP_HOOK_VLAN_SET
+ * @see LWIP_HOOK_VLAN_SET
  *
  * @param netif the lwIP network interface on which to send the packet
  * @param p the packet to send. pbuf layer must be @ref PBUF_LINK.

src/netif/lowpan6.c

@@ -611,7 +611,7 @@ lowpan4_output(struct netif *netif, struct pbuf *q, const ip4_addr_t *ipaddr)
  * @param q The pbuf(s) containing the IP packet to be sent.
  * @param ip6addr The IP address of the packet destination.
  *
- * @return
+ * @return err_t
  */
 err_t
 lowpan6_output(struct netif *netif, struct pbuf *q, const ip6_addr_t *ip6addr)

src/netif/ppp/pppos.c

@@ -404,9 +404,9 @@ pppos_destroy(ppp_pcb *ppp, void *ctx)
 #if !NO_SYS && !PPP_INPROC_IRQ_SAFE
 /** Pass received raw characters to PPPoS to be decoded through lwIP TCPIP thread.
  *
- * @param pcb PPP descriptor index, returned by pppos_create()
- * @param data received data
- * @param len length of received data
+ * @param ppp PPP descriptor index, returned by pppos_create()
+ * @param s received data
+ * @param l length of received data
  */
 err_t
 pppos_input_tcpip(ppp_pcb *ppp, u8_t *s, int l)
@@ -458,9 +458,9 @@ PACK_STRUCT_END
 
 /** Pass received raw characters to PPPoS to be decoded.
  *
- * @param pcb PPP descriptor index, returned by pppos_create()
- * @param data received data
- * @param len length of received data
+ * @param ppp PPP descriptor index, returned by pppos_create()
+ * @param s received data
+ * @param l length of received data
  */
 void
 pppos_input(ppp_pcb *ppp, u8_t *s, int l)

src/netif/slipif.c

@@ -304,7 +304,7 @@ slipif_rxbyte(struct netif *netif, u8_t c)
 /** Like slipif_rxbyte, but passes completed packets to netif->input
  *
  * @param netif The lwip network interface structure for this slipif
- * @param data received character
+ * @param c received character
  */
 static void
 slipif_rxbyte_input(struct netif *netif, u8_t c)