Update specification of _splice_at.

It is more interressing to have the iterator of the target list
to advance too. It allows copying items from list to list in the same order.

README.md

@@ -791,7 +791,8 @@ After wise, 'it' points to the next element of 'list2'.
 ##### void name\_splice\_at(name\_t list1, name\_it\_t it1, name\_t list2, name\_it\_t it2)
 
 Move the element pointed by 'it2' (which is an iterator of 'list2') from the list 'list2' to the position just after 'it1' in the list 'list1'.
-After wise, 'it2' points to the next element of 'list2'.
+After wise, 'it2' points to the next element of 'list2'
+and 'it1' points to the inserted element in 'list1'.
 If 'it1' is the end position, it inserts it at the first element (just like \_insert\_at).
 
 ##### void name\_splice(name\_t list1, name\_t list2)