Pass abc RPC argument descriptions to RPCHelpMan

Summary:
This tree in favor of D5571.

Partial backport of Core PR14796

Test Plan:
On master:

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

On the patched branch:

ninja
./bitcoind
./getrpchelps.sh RPCs abcPatched
./checkrpc.sh RPCs master abcPatched D5700diff

Verify the changes in D5700diff.

Scripts:

D5700diff:

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5700