Pass rpc/blockchain RPC argument descriptions to RPCHelpMan

Summary: Partial backport of Core PR14796

Test Plan:

mkdir master
mkdir patched

On master:

ninja
./bitcoind
./bitcoin-cli help > RPCs
./getrpchelps.sh RPCs master

On the patched branch:

ninja
./bitcoind
./getrpchelps.sh RPCs patched
./checkrpc.sh RPCs master patched D5716diff

Verify the changes in D5716diff.

Scripts:

D5716diff:

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5716