[Vertex AI] Add documentation for Imagen symbols

[andrewheard]

• Added DocC comments to all public symbols.
• Added a notice about the Imagen feature being a Public Preview.
• Fixed the casing of gcsURI in generateImages(prompt:gcsURI:) (was gcsUri) to match the API proposal and Swift style guide.

#14221
#no-changelog

[paulb777]

Nice API docs.

Is there a good way to get this level of quality consistently across all platforms without too much duplicate work?

[andrewheard]

Nice API docs.

Is there a good way to get this level of quality consistently across all platforms without too much duplicate work?

Thanks!

I don't think we'd be able to make them consistent for this release without duplicate work since the rest of the SDKs have already added docs. Fortunately I was able to grab from the others (and Cloud or Developer API docs) and then elaborated where I saw fit. Something we could consider in the future is adding docs to one set of API signatures in a Firebase API Proposal and then, once approved, each platform owner can copy/paste them and make the slight language-specific formatting changes. That would allow us to iterate on the docs together, might make the proposals easier to read since the docs would provide context/detail about the API surface (but may be too noisy, especially with Markdown and links) and reduce some duplicate work. That's just one idea though, we should probably bring it up in one of our meetings. cc: @rachelsaunders in case she has ideas too (and for visibility on the docs).