Document version/release note policy for C++ level

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Yeah, we have a tendency to forget to add C++ stuff to the release notes. But we should be doing it for public methods.

We also, confusingly, don't often count them towards the version number.
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_Originally posted by @arcondello in https://github.com/dwavesystems/dwave-optimization/issues/497#issuecomment-4208157464_