list: Clarify description of list_moved().

This function doesn't have any users, but an upcoming commit will add one.

lib/list.c

@@ -90,7 +90,11 @@ list_replace(struct list *element, const struct list *position)
 }
 
 /* Adjusts pointers around 'list' to compensate for 'list' having been moved
- * around in memory (e.g. as a consequence of realloc()). */
+ * around in memory (e.g. as a consequence of realloc()).
+ *
+ * This always works if 'list' is a member of a list, or if 'list' is the head
+ * of a non-empty list.  It fails badly, however, if 'list' is the head of an
+ * empty list; just use list_init() in that case. */
 void
 list_moved(struct list *list)
 {