Help me understand the associations `on_replace: :delete` example in the docs

In Ecto.Changeset — Ecto v3.14.0 there is a part which says:

The :delete option in particular must be used carefully as it would allow users to delete any associated data. If you need deletion, it is often preferred to add a separate boolean virtual field to the changeset function that will allow you to manually mark it for deletion, as in the example below: […]

defmodule Comment do
  use Ecto.Schema
  import Ecto.Changeset

  schema "comments" do
    field :body, :string
    field :delete, :boolean, virtual: true
  end

  def changeset(comment, params) do
    cast(comment, params, [:body, :delete])
    |> maybe_mark_for_deletion
  end

  defp maybe_mark_for_deletion(changeset) do
    if get_change(changeset, :delete) do
      %{changeset | action: :delete}
    else
      changeset
    end
  end
end

However, this example is incomplete and I am having trouble applying that.

Carrying on with the comment theme, If I have this as the schema that inserts it with cast_assoc, where can I mark it as deleted? :

(I know it’s weird to have one comment but that’s the type of relationship I’m using in my actual code, so I want to keep the example similar)

defmodule Post do
  use Ecto.Schema
  import Ecto.Changeset

  schema "posts" do
    field(:title, :string)
    field(:body, :string)
    has_one(:comment, Comment)
  end

  def update_changeset(post, attrs) do
    post
    |> Repo.preload(:comment)
    |> cast(attrs, [:title, :body])
    |> cast_assoc(:comment, required: true, on_replace: :nilify)
  end
end