[backport#16071] RPC: Hint for importmulti in help output of importpubkey and importaddress

Summary:
RPC: Hint for importmulti in help output of importpubkey and importaddress (Kristaps Kaupe)

Pull request description:

Similar to #12702. Hint for `importmulti` also in help output of `importpubkey` and `importaddress`.

Backport of Core PR16071

Test Plan:

ninja
bitcoin-cli help importpubkey|importaddress, check it looks good

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7366