docs: document workflow token behaviors

[WilliamBergamin]

Assumptions that bolt makes around workflow token and how they are used is not documented, this can lead to some confusion. I think we should document that whenever a workflow token is found in an event payload it will be used by the client injected into the handler. It is also worth documenting that this behavior can be disabled with attachFunctionToken = false

List any relevant URLs

https://tools.slack.dev/bolt-js/concepts/custom-steps

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[zimeg]

📝 While this is still fresh in mind, we might want to also include notes of how this token is available in context:

https://tools.slack.dev/bolt-js/concepts/context

A potential change of #2513 might also be applied to this, but that's not so certain at the moment 🫡

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