a fix to the discriminated union docs to make it clearer what operations we do and don't support
Due to various challenges with TypeScript generics, we can’t make the $jazz.ensureLoaded, $jazz.set and $jazz.subscribe methods typesafe on discriminated unions. This was causing warnings for one of our adopters in their project.
Although this was previously covered in the docs, it was hidden towards the bottom of the page.
This PR makes it clearer and more prominent.