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

Inizia dalla documentazione del modello dati di Python e rivedi la descrizione esistente di function.closure insieme a co_freevars. Chiarisci che compaiono solo i nomi a cui fa riferimento la funzione interna e conferma la formulazione con il team della documentazione prima di procedere; il lavoro è completato quando il comportamento è esplicito nella sezione pertinente.