Fix and simplify newlines in doxygen documentation

Replace '\n' with '<br>', as this allows doxygen to understand reference
names followed by newline. For some cases just drop the newline if it's
not required.

Doxygen 1.8.15 doesn't like if the name of reference is followed by
anything else than (selected?) punctuation or whitespace.

bug #56004

src/netif/ethernet.c

@@ -67,7 +67,7 @@ const struct eth_addr ethzero = {{0, 0, 0, 0, 0, 0}};
  * @ingroup lwip_nosys
  * Process received ethernet frames. Using this function instead of directly
  * calling ip_input and passing ARP frames through etharp in ethernetif_input,
- * the ARP cache is protected from concurrent access.\n
+ * the ARP cache is protected from concurrent access.<br>
  * Don't call directly, pass to netif_add() and call netif->input().
  *
  * @param p the received packet, p->payload pointing to the ethernet header

src/netif/slipif.c

@@ -46,12 +46,12 @@
  * This is an arch independent SLIP netif. The specific serial hooks must be
  * provided by another file. They are sio_open, sio_read/sio_tryread and sio_send
  *
- * Usage: This netif can be used in three ways:\n
- *        1) For NO_SYS==0, an RX thread can be used which blocks on sio_read()
- *           until data is received.\n
- *        2) In your main loop, call slipif_poll() to check for new RX bytes,
- *           completed packets are fed into netif->input().\n
- *        3) Call slipif_received_byte[s]() from your serial RX ISR and
+ * Usage: This netif can be used in three ways:
+ *        1. For NO_SYS==0, an RX thread can be used which blocks on sio_read()
+ *           until data is received.
+ *        2. In your main loop, call slipif_poll() to check for new RX bytes,
+ *           completed packets are fed into netif->input().
+ *        3. Call slipif_received_byte[s]() from your serial RX ISR and
  *           slipif_process_rxqueue() from your main loop. ISR level decodes
  *           packets and puts completed packets on a queue which is fed into
  *           the stack from the main loop (needs SYS_LIGHTWEIGHT_PROT for