add documentation

include/network_util.h

@@ -10,8 +10,14 @@
 typedef struct connection connection_t;
 
 /**
- * TODO: put docs here
- **/
+ * A callback function type used for handling network events.
+ * 
+ * @param fd The file descriptor of the connection
+ * @param str The string data received from the connection
+ * 
+ * This callback is typically used with the nu_multiplex function to handle
+ * data received from local or remote connections.
+ */
 typedef void nu_callback(int fd, char *str);
 
 /**