Document that remove of Singly/DoublyLinkedList relies on the node being in the list

lib/std/DoublyLinkedList.zig

@@ -105,6 +105,7 @@ pub fn prepend(list: *DoublyLinkedList, new_node: *Node) void {
 }
 
 /// Remove a node from the list.
+/// Assumes the node is in the list.
 ///
 /// Arguments:
 ///     node: Pointer to the node to be removed.

lib/std/SinglyLinkedList.zig

@@ -85,6 +85,8 @@ pub fn prepend(list: *SinglyLinkedList, new_node: *Node) void {
     list.first = new_node;
 }
 
+/// Remove `node` from the list.
+/// Asserts that `node` is in the list.
 pub fn remove(list: *SinglyLinkedList, node: *Node) void {
     if (list.first == node) {
         list.first = node.next;