net: if: Documentation missing for IPv6 DAD start time variable

Add documentation for DAD start time variable as it was missing.

Signed-off-by: Jukka Rissanen <[email protected]>

Author: jukkar

include/zephyr/net/net_if.h

@@ -94,6 +94,8 @@ struct net_if_addr {
 		struct {
 			/** Duplicate address detection (DAD) timer */
 			sys_snode_t dad_node;
+
+			/** DAD start time */
 			uint32_t dad_start;
 
 			/** How many times we have done DAD */