Missing docs for ES|QL sorting by expressions

In the work in https://github.com/elastic/elasticsearch/pull/107158 ES|QL was given the ability to allow sorting by expressions. However, this was never documented, other than in the [Release notes for 8.14.0](https://www.elastic.co/guide/en/elasticsearch/reference/current/release-notes-8.14.0.html#feature-8.14.0). We should update the ES|QL docs to include this feature.
