Updates to docs

[LostKobrakai]

This touches two topics, but it's just small changes I hope that's ok.

• Mention start_owner! in the module docs. I personally would consider this the better default, but I'm not sure we want to go there for the docs.
• Explicitly mention caller tracking for allowance in manual mode, so it's clear that this does allow for async tests.

[josevalim]

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