doc: Include main Network APIs doxygen documentation

Added missing dogygen definitions.

Jira: ZEP-1495

Change-Id: Ibc087f3f174b8af45b0726c0d82b77feff36d4e3
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>

include/net/net_if.h

@@ -12,6 +12,12 @@
 #ifndef __NET_IF_H__
 #define __NET_IF_H__
 
+/**
+ * @brief Network Interface abstraction layer
+ * @defgroup net_if Network Interface abstraction layer
+ * @{
+ */
+
 #include <device.h>
 #include <misc/slist.h>
 
@@ -1076,6 +1082,13 @@ struct net_if *net_if_get_by_index(uint8_t index);
  */
 uint8_t net_if_get_by_iface(struct net_if *iface);
 
+/**
+ * @typedef net_if_cb_t
+ * @brief Callback used while iterating over network interfaces
+ *
+ * @param iface Pointer to current network interface
+ * @param user_data A valid pointer on some user data or NULL
+ */
 typedef void (*net_if_cb_t)(struct net_if *iface, void *user_data);
 
 /**
@@ -1157,4 +1170,8 @@ struct net_if_api {
 }
 #endif
 
+/**
+ * @}
+ */
+
 #endif /* __NET_IF_H__ */