Use Bitcoin Core-style ACK / NACK / Nit language in pull request reviews

From https://bitcoincore.org/en/faq/contributing-code/#peer-review:

> Anyone may participate in peer review which is expressed by comments in the pull request. Typically reviewers will review the code for obvious errors, as well as test out the patch set and opine on the technical merits of the patch. Project maintainers take into account the peer review when determining if there is consensus to merge a pull request (remember that discussions may have been spread out over github, mailing list and IRC discussions). The following language is used within pull-request comments:
>
> - `ACK` means “I have tested the code and I agree it should be merged”;
> - `NACK` means “I disagree this should be merged”, and must be accompanied by sound technical justification. NACKs without accompanying reasoning may be disregarded;
> - `utACK` means “I have not tested the code, but I have reviewed it and it looks OK, I agree it can be merged”;
> - `Concept ACK` means “I agree in the general principle of this pull request”;
> - `Nit` refers to trivial, often non-blocking issues.

Start by reading the linked Bitcoin Core contributing FAQ and the repository's existing styleguide. Identify where pull-request review conventions belong; done means the styleguide clearly documents the ACK, NACK, utACK, Concept ACK, and Nit meanings stated in the issue.

Written by the indexing model from the issue text.