Pass rpc/rawtransaction 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 D5720diff.txt

Verify the changes in D5720diff.txt.

Scripts:

D5720diff:

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D5720