Clarify which variables appear in function.__closure__

While working with nested functions, I observed that function.__closure__ only
contains cells for variables that are actually referenced inside the inner
function body. Variables defined in the enclosing scope but not used in the
inner function do not appear in co_freevars and therefore do not appear in
function.__closure__.

Example:

def outer():
    x = 1
    y = 2
    z = 3
    def inner():
        return x
    return inner

inner.__closure__  # contains only x

This behavior matches the execution model (“free variables”), but the current
documentation only states that __closure__ contains bindings for names listed
in co_freevars. It does not explicitly state that only referenced names are
included.

This PR proposes adding a brief clarification to make this behavior explicit.

Before continuing with the pull request, I would like confirmation from the
documentation team whether this detail should be included in the data model
section.

Linked PRs

• gh-142086

Python データモデルのドキュメントから始め、co_freevars と併せて function.closure の既存の説明を確認します。内側の関数が参照する名前だけが含まれることを明確にし、先に進む前にドキュメントチームと文言を確認します。関連するセクションで動作が明示されていれば完了です。