Follow-up to PR #16521.
- Fix incorrect value in rpc_rawtransaction test as per https://github.com/bitcoin/bitcoin/pull/16521/files#r325842308
- Improve the code docs
- Use named arguments as per https://github.com/bitcoin/bitcoin/pull/16521/files#r310715127
Happy to squash or keep only the first commit if the others are too fixup-y.