[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