docs(transformers): clarify that shiki does not parse meta

[olets]

Description

Currently the docs imply that Shiki provides a parsed meta object. This change removes that implication. This change also provides an example of dashcased meta attribute keys and quoted meta attribute values.

Linked Issues

• options.meta does not return parsed metadata object in transformer #792

Additional context

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[cworld1]

If you want to help the reader more, I think it is very appropriate to provide a link to the issue #792 here, at least to let the reader know that there is a solution that may be helpful.