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

Summary:
Final step of [[https://github.com/bitcoin/bitcoin/pull/14468 | 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

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