#[turbo_tasks::value(transparent)]: Generate docs & fail on invalid callsites (#8087)
### Description
Most `#[turbo_tasks::value(transparent)]` types leave their inner value as private. I think this okay, but because rustdoc hides private fields by default, it makes it hard to understand at a glance the contained value, without scrolling down to the `impl` of `VcValueType`. This PR generates documentation for these.

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<summary>Also checked that it works if there's an existing doc comment, extending rather than replacing the existing documentation.</summary>

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**Alternative:** We could fail if the field is non-`pub`, and update the callsites. Let me know if this is preferred. The contained fields are *basically* public anyways, as they can be accessed via `Vc`'s APIs.
While modifying this code, I realized that we don't generate an error if `#[turbo_tasks::value(transparent)]` would be a no-op. The second commit in this PR adds an error and updates the callsites. This also exposed some issues in the next.js repository, which are fixed in https://github.com/vercel/next.js/pull/65337 .
### Testing Instructions
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Closes PACK-3038