rpc: Document an RPCResult for all calls; Enforce at compile time

Summary:
Also remove unneeded whitespaces in rpcwallet.cpp

This is a backport of Core PR18346

Test Plan:

ninja all check-all
grep -R ../RPCResults

Display the help message for some commands (e.g. src/bitcoin-cli help getavalanchekey)

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D8845