[docs] [issue33722] Document builtins in mock_open

[Karthikeyan Singaravelan]

Karthikeyan Singaravelan <tir.karthi at gmail.com> added the comment:

The example looks like a good addition to me since mocking builtins.open is not documented for this use case. Please start a new PR.

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nosy: +xtreak

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