Some documentation updates and fixes

Dedicated Zero-copy RX page
Fix doxygen warnings

doc/ZeroCopyRx.c

@@ -0,0 +1,39 @@
+typedef struct my_custom_pbuf
+{
+   struct pbuf_custom p;
+   void* dma_descriptor;
+} my_custom_pbuf_t;
+
+LWIP_MEMPOOL_DECLARE(RX_POOL, 10, sizeof(my_custom_pbuf_t), "Zero-copy RX PBUF pool");
+
+void my_pbuf_free_custom(void* p)
+{
+  my_custom_pbuf_t* my_puf = (my_custom_pbuf_t*)p;
+
+  LOCK_INTERRUPTS();
+  free_rx_dma_descriptor(my_pbuf->dma_descriptor);
+  LWIP_MEMPOOL_FREE(RX_POOL, my_pbuf);
+  UNLOCK_INTERRUPTS();
+}
+
+void eth_rx_irq()
+{
+  dma_descriptor*   dma_desc = get_RX_DMA_descriptor_from_ethernet();
+  my_custom_pbuf_t* my_pbuf  = (my_custom_pbuf_t*)LWIP_MEMPOOL_ALLOC(RX_POOL);
+
+  my_pbuf->p.custom_free_function = my_pbuf_free_custom;
+  my_pbuf->dma_descriptor         = dma_desc;
+
+  invalidate_cpu_cache(dma_desc->rx_data, dma_desc->rx_length);
+  
+  struct pbuf* p = pbuf_alloced_custom(PBUF_RAW,
+     dma_desc->rx_length,
+     PBUF_REF,
+     &my_pbuf->p,
+     dma_desc->rx_data,
+     dma_desc->max_buffer_size);
+
+  if(netif->input(p, netif) != ERR_OK) {
+    pbuf_free(p);
+  }
+}

doc/doxygen/main_page.h

@@ -100,6 +100,12 @@
  * https://savannah.nongnu.org/bugs/?group=lwip
  */
 
+/**
+ * @page zerocopyrx Zero-copy RX
+ * The following code is an example for zero-copy RX ethernet driver:
+ * @include ZeroCopyRx.c
+ */
+
 /**
  * @defgroup lwip_nosys Mainloop mode ("NO_SYS")
  * @ingroup lwip