[docs] [issue36490] Modernize function signature format in Archiving section of shutil doc

[Inada Naoki]

Inada Naoki <songofacandy at gmail.com> added the comment:

I don' think they are modern-style vs legacy-style.

When explicit default value is present, the default value is documented in signature like `param=42`.
On the other hand, when default value is not explicit (just an placeholder), the parameter is expressed using optional group `[param]`.

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nosy: +inada.naoki

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