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

從 Python 資料模型文件開始，搭配 co_freevars 檢視現有的 function.closure 說明。明確指出只有內部函式參照的名稱才會出現，並在繼續之前與文件團隊確認措辭；相關區段明確說明此行為即表示完成。