Augment C2178 error reference with consteval

[Rageking8]

• Merge and condense mutable remarks to make it clearer and give space to the new consteval remarks
• Clean up existing mutable example
• Add new remarks and example for consteval
  • The "Example: ..." heading is used instead of nested "Examples" heading as it adds the various examples to the "In this article" UI for quick navigation. It's especially useful for error/warning references with lots of example entries.
  • Here is the relevant quote in the draft spec:
    https://github.com/cplusplus/draft/blob/c3bd02a2e44c53c8fe7ba5b48a94bacc9186235d/source/declarations.tex#L943-L944
• Update ms.date metadata

I recommend to review one commit at a time.

[prmerger-automator]

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PRMerger Results

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[Court72]

@TylerMSFT

Can you review the proposed changes?

Important: When the changes are ready for publication, adding a #sign-off comment is the best way to signal that the PR is ready for the review team to merge.

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