Hi Nikolay,
2.x will support API docs much like 1.x, with the exception that the team isn't planning to support on-the-fly analysis like iron-component-page does in 1.x, which was slow and problematic (although very convenient).
For 2.x, you'll need a build step to run polymer-analyzer on your elements and dump a JSON file that can be read by iron-doc-viewer. We may also end up adding some functionality into the CLI (for example, polymer serve could auto-generate docs).
I don't think doc tooling is going to be ready to ship with 2.x, but hopefully we'll be able to supply some tools and docs in the near future. Until then, hope this helps.
Thanks,
Arthur