API documentation for updating preferences is missing

There is a feature in the API to [update the current user’s preferences](https://github.com/hypothesis/h/pull/4350), but this is missing from the [relevant section of the API reference documentation](https://github.com/hypothesis/h/blob/a18b8094eaf7b54b81823cf04511bf236a51f59b/docs/_extra/api-reference/hypothesis.yaml#L150-L170). We should fix this, so the API and the documentation are more consistent.