Update lib/server/undo_server_observer_behaviour.ex

Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

lib/server/undo_server_observer_behaviour.ex

@@ -4,6 +4,23 @@ defmodule Yex.UndoServer.ObserverBehavior do
   stack items during undo/redo operations.
   """
 
+  @doc """
+  Called when a new item is about to be added to the undo stack.
+
+  ## Parameters
+  - stack_item: A map containing the undo operation details
+
+  ## Returns
+  - `{:ok, modified_item}` to add the possibly modified item to the stack
+  - `:ignore` to prevent the item from being added to the stack
+  """
   @callback handle_stack_item_added(stack_item :: map()) :: {:ok, map()} | :ignore
+
+  @doc """
+  Called after an item has been popped from the undo stack during undo/redo operations.
+
+  ## Returns
+  - `:ok` to acknowledge the pop operation
+  """
   @callback handle_stack_item_popped() :: :ok
 end