docs: add note about fetching tags during release

[jbedard]

So I don't forget next time...

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[jbedard]

FWIW the issue prompting this was me tagging + pushing a release without first doing git fetch --tags first. So the generated locally was not up to date, but the tag generated by CI for the binaries (but not the release notes) was correct (see #766).

Why does CI generate the tag again? I'm not sure if that's worth investigating or fixing now, but fyi.