message: test: doc: improve doc for chain_transaction() helper
  Change to docstring format and describe the functions
  purpose, its parameters and return value in more detail.
diff: @@ -481,9 +481,13 @@ def create_confirmed_utxos(fee, node, count):
  return utxos
  -# Build a transaction that spends parent_txid:vout
  -# Return amount sent
  def chain_transaction(node, parent_txids, vouts, value, fee, num_outputs):
  + """Build and send a transaction that spends the given inputs (specified
  + by lists of parent_txid:vout each), with the desired total value and fee,
  + equally divided up to the desired number of outputs.
  +
  + Returns a tuple with the txid and the amount sent per output.
  + """
  send_value = satoshi_round((value - fee)/num_outputs)
  inputs = []
  for (txid, vout) in zip(parent_txids, vouts):
  