Current doc example:

with nested(A, B, C) as (X, Y, Z):
    do_something()

with A as X:
    with B as Y:
        with C as Z:
            do_something()

Recommended docs change:

with nested(A(), B(), C()) as (X, Y, Z):
    do_something()

m1, m2, m3 = A(), B(), C()
with m1 as X:
    with m2 as Y:
        with m3 as Z:
            do_something()

This makes it clearer that when using nested, the context managers are
all created outside the scope of the with statement.