[rpc] add 'getnewaddress' hint to 'generatetoaddress' help text.

Summary:
Final step of PR14468

Depends on D5971

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D5972