RPC: Rephrase generatetoaddress help, and use PACKAGE_NAME

Summary: This is a backport of core#19282

Test Plan: src/bitcoin-cli help generatetoaddress

Reviewers: #bitcoin_abc, Fabien, deadalnix

Reviewed By: #bitcoin_abc, Fabien, deadalnix

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9615