chore: replace informative with non-normative for consistency

[Bashamega]

fixes #914

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[Bashamega]

Done @frivoal :)

[frivoal]

Thank you @Bashamega. Help improving the quality and consistency of the text is always very much appreciated!

[TallTed]

s/an non-normative/a non-normative/
:-)

[nigelmegitt]

Thanks for addressing this @Bashamega - please could you also address the terminology in the Document Conventions section, and the example within it?

I also just noticed that the non-normative references, which are generated by tooling, are in a section called "Informative references" - I don't know if that can be changed, but in #914 when we discussed the preference for non-normative vs informative, that would have been a useful data point!

The deliberate cases where "informative" were left are now looking quite confusing to me. Perhaps we should replace them with "informational" or something else that makes it clear that it is not a synonym for "non-normative"?

[frivoal]

@nigelmegitt The other parts are not coming from the source code, but from bikeshed (speced/bikeshed#3077). I'll follow up to get that addressed there, but this PR is a net improvement.

As for the remaining instances of informative in the text, I think they're actually quite natural and better than alternative rephrasing (like "informational"). Unless you're specifically looking for them as we are here, I think it's unlikely anybody would notice anything unusual about them.