Update docs

[wojtekmach]

• handle_declare no longer can return {:ok, cursor, state}
• handle_prepare no longer can return {:ok, result, state}
• handle_rollback can return any status
• Re-use status type in handle_status

[wojtekmach]

There's at least one other callback that needs update, I'll re-open when done.

[wojtekmach]

ready!

[josevalim]

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