rpc: Fix documentation assertion for `getrawtransaction`
rpc: Fix documentation assertion for `getrawtransaction`

When `getrawtransaction` is successfully used on a coinbase transaction,
there is an assertion error. This is very unlikely but happens in the
test in #24358.

This does the following:

- Add missing "coinbase" documentation.

- Synchronize documentation between `getrawtransaction` and
  `decoderawtransaction`, the two users of `TxToUniv` that have detailed
  documentation. `decodepsbt` also uses it but fortunately elides this block.

- Change "vout[].amount" to `STR_AMOUNT` for consistency.

- Add maintainer comment to keep the two places synchronized. It might
  be possible to get smarter with deduplication, but there are some
  extra fields that prevent the obvious way.

Github-Pull: #24716
Rebased-From: 71038a151e3136c22449e1a5cb1f386e8474c32d

bitcoinknots/bitcoin,bitcoinknots/bitcoin,bitcoinknots/bitcoin,bitcoinknots/bitcoin,bitcoinknots/bitcoin