Markdown Conversion of HTML `hostname` option requires URL

### Existing documentation URL(s)

- https://developers.cloudflare.com/workers-ai/features/markdown-conversion/conversion-options/#html
- https://developers.cloudflare.com/workers-ai/features/markdown-conversion/how-it-works/#html

### What changes are you suggesting?

The description of the Markdown Conversion Options (e.g. for the `env.AI.toMarkdown` binding) describes the HTML `hostname` option as:
> `hostname`: string to use as a host when resolving relative links inside the HTML.

The property name and this description suggests that a bare `example.com` style hostname (e.g. the result of `URL.hostname`) should be used. However, that leaves relative links in the markdown result (if there is no `` element in the HTML itself).

It appears that this option actually requires a base URL, which will typically be the full URL of the page being converted (`URL.origin` wouldn't suffice for pages in sub-directories). The How It Works page hints at this, but it should be explicit in the description of the Conversion Options, especially since the option name itself is misleading.

### Additional information

_No response_