Arvados

After #19929, every schema, property, method, and parameter in the Arvados API discovery document has a description, and we have a test to ensure it stays that way.

In principle, we could generate much of the API reference from the discovery document now. This would have all the usual benefits of having a single source of truth: the web reference couldn't get out-of-date or conflict with the discovery document and all the SDK documentation generated from it.

Issues we would need to solve to do this:

• The discovery document documentation is written in Markdown, because that's best-supported by our different SDK consumers. Currently our API documentation is in Textile. Need to figure out a way to bridge this gap.
• There are some situations where the web API documentation will expound a lot of detail about a particular property or method because that's the best place to do it. We should keep this information and figure out a way to do it. Note this might be doable just with some tweaks to the structure of the web API documentation. It doesn't necessarily need a programmatic solution.
• The web API reference tables sometimes include different example values to illustrate a property or parameter. There's currently not a place for these in the discovery document. If we want to keep them, we'll have to figure out a way to do that.