Fix nits in RPC help messages
Summary: Fix some nits found while reviewing D5239.
Test Plan:
ninja all check ./src/bitcoind -daemon ./src/bitcoin-cli help signrawtransactionwithkey ./src/bitcoin-cli help listunspent ./src/bitcoin-cli help signrawtransactionwithwallet ./src/bitcoin-cli help walletcreatefundedpsbt
Make sure the help is correct.
Reviewers: #bitcoin_abc, markblundeberg
Reviewed By: #bitcoin_abc, markblundeberg
Differential Revision: https://reviews.bitcoinabc.org/D5255