Codeception\Util\Stub documentation invalid

`Codeception\Util\Stub::atLeastOnce()`, `Codeception\Util\Stub::exactly()`, `Codeception\Util\Stub::never()`, `Codeception\Util\Stub::once()` examples do not show, that in order for those methods to work, you have to insert `\Codeception\Test\Unit`

Locate the documentation examples for Codeception\Util\Stub::atLeastOnce(), exactly(), never(), and once(). Review how the examples reference Codeception\Test\Unit, then update them so the required setup is explicit and verify that each example is consistent.

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