Amend formatting of `Substring.utf8Span` example code.
Use DocC tables in `Unicode.UTF8.ValidationError` overview.
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Co-authored-by: Alex Martini <amartini@apple.com>
The implementation previously took advantage of a missing diagnostic. They
returned a borrowed Span with a dependency on a copied argument. This illegally
promotes the dependency. This is what the author intended to do (the
implementation was correct) but the compiler can't know that so the code needs
to be explicit about overriding the lifetime.
