rpc: Add HelpExampleRpcNamed and HelpExampleCliNamed and use them for createwallet doc

Summary:
This is a backport of core#21302

Depends on D12775

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12776