ensure links only belong to 1 list

also some small documentation changes.

include/nickel/linkedlist.h

@@ -47,9 +47,13 @@ static inline ni_list_node* ni_list__get_back(ni_list* l) {
 
 /* Iter Helpers */
 
+// helper macro for element traversal, ordering of operations may cause seg faults
+// generally just don't remove 'it', this causes it->next on the next loop to segfault
 #define NI_LIST__FOREACH(lptr) \
     for (ni_list_node* it = (lptr)->head.next; it != &(lptr)->head; it = it->next)
 
+// helper macro for element traversal, that also makes a copy of it in tmp
+// useful for removing/moving the current node (it)
 #define NI_LIST_FOREACH_SAFE(tmp, lptr) \
     for (ni_list_node* it = (lptr)->head.next, *tmp = it->next; \
         it != &(lptr)->head; \