Add logpath description for getrpcinfo help

Summary:
Backport of Core PR16332
Depends on D7820

Test Plan: Build, run src/bitcoind and src/bitcoin-cli help getrpcinfo

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7821