Fix docs for Target.kind and Target.crate_types

[Shnatsel]

I only realized that far more values than documented are possible in the Target.kind fields after my code broke for end users.

This brings the crate doc commends in line with the official documentation on cargo metadata.

The docs for kind are based on both the cargo metadata documentation and my experiments, while the docs for crate_types are based on documentation alone.

[jplatte]

(not a maintainer)

There should probably be a newline in the docs between the first sentence and the rest for both fields (so that it renders as two paragraphs).

[oli-obk]

We should probably make these a non_exhaustive enum.

For now, please also document that new variants may get added at any time

[Shnatsel]

Yes, I agree that an enum would be better. But that would break semver, and I wanted to start with a less invasive change.

[Shnatsel]

I've addressed all review feedback, PTAL.