Start passing some wallet/rpcwallet RPC argument descriptions to RPCHelpMan

Summary:
This is the first of two patches resulting from breaking the changes
to rpcwallet.cpp into smaller parts.

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 rpcwallethelp.txt

Verify the changes in rpcwallethelp.txt.

Scripts:

D5722diff:

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5732