rpc: Make rpc documentation not depend on rpc args

Summary:

This is required to host the documentation on a static resource (like a website or pdf)

Thisis a backport of Core PR18499

Test Plan:
Display the help message of one of the affected commands:

for COMMAND in sendtoaddress signmessage sendmany keypoolrefill signrawtransactionwithwallet sethdseed walletprocesspsbt
do
    src/bitcoin-cli help $COMMAND
done

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D8902