Clarify that variadic positional parameters can be expressed with Callable

[Daraan]

As @JelleZijlstra noted here the current notion of

The Callable form provides no way to specify [...] variadic parameters [...]

is not correct as we can express variadic positional parameters with an unpacked tuple, e.g. Callable[[*tuple[int, ...]], str].
This PR adds clarification for this case.

I've removed the notion about variadics completely in that paragraph, an alternative would be

no way to specify keyword-only parameters, variadic keyword-only parameters, or ..."

which feels redundant. Feel free to share other suggestions