English Google searches about
bitcoin-cli often lead to two sites:
Any idea which one is the official one, endorsed by the maintainers?
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As far as I know there is no official documentation.
The closest you can get to it is the combination of the GitHub: https://github.com/bitcoin/bitcoin
With the whitepaper: https://bitcoincore.org/bitcoin.pdf
Changelog appears with every version on the core site, e.g.: https://bitcoincore.org/en/releases/0.12.1/
And lastly, while I already linked to GitHub it is probably worth mentioning that open and closed issues are documented there: https://github.com/bitcoin/bitcoin/issues?utf8=%E2%9C%93&q=
Hope this helps.