Add note to schemas tutorial about the date failure (and general unreliability of LLMs)

In https://github.com/simonw/llm/blob/0.23/docs/schemas.md

https://bsky.app/profile/dracos.co.uk/post/3ljayeoea2224

The published date on that first AP article is 27th February, the last updated is 28th February, yet your output in that tutorial has 26th February. The date on the Katy Perry article is totally wrong.

On investigation, it turns out running AP content through strip-tags removes the dates entirely thanks to their web component:

After strip-tags that ends up looking like this:

By 
JANIE HAR
Updated [hour]:[minute] [AMPM] [timezone], [monthFull] [day], [year]

This is a great opportunity to explain that you need to check this stuff thoroughly, but I don't know if I want to explain Web Components as part of the tutorial!

Start with docs/schemas.md and review the linked Bluesky report and the shown strip-tags output. Add a note explaining that the tutorial's extracted dates can be wrong or missing and that LLM output requires careful verification; done means the warning is clear without requiring a Web Components explanation.