genAI docs: "bookmarked state" links to `Session` docs

[gadenbuie]

In https://shiny.posit.co/py/docs/genai-chatbots.html#bookmark-messages

When a Shiny app reloads, the app returns to it’s original state, unless the URL includes bookmarked state.

The link points to express.session.Session which doesn't include any information about bookmarking.

cc @cpsievert