Fix docs on `Psbt::extract_tx`

Our API docs mirror the Rust side, but our errors don't, making the API docs incorrect. We should either:

1. Fix the docs
2. See if we can expand the error to include the data required to make the docs valid!

Start at the Psbt::extract_tx API documentation and compare it with the Rust-side documentation and the current error returned by the API. Determine whether the documented behavior or the error data needs to change; done means the API docs accurately describe the available error information.

Written by the indexing model from the issue text.