[backport#16285] rpc: Improve scantxoutset response and help message

Summary:
qa: Check scantxoutset result against gettxoutsetinfo (João Barbosa)
rpc: Improve scantxoutset response and help message (João Barbosa)

Pull request description:

The new response keys `height` and `bestblock` allow the client to know at what point the scan took place.

The help message now has all the response keys (`result` and `txouts` were missing) and it's improved a bit. Note that `searched_items` key is renamed to `txouts`, considering `scantxoutset` is marked experimental.

Backport of Core PR16285

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7402