Docs for 'an idempotent worker' to use `it_behaves_like`

What does this MR do and why?

This MR changes the docs examples for the use of the 'an idempotent worker' shared example to use it_behaves_like.

it_behaves_like creates a context around the shared examples, whereas include_examples does not. When using 'an idempotent worker' with include_examples, the subject from the shared examples will leak into the calling spec context, as it will redefine what subject is.

This results in the subject outside of the shared example group becoming a worker that has its #perform method called twice.

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