We at @ahrefs use have use messages descriptions quite often, and lack of description support was the blocker to migrate from object message type to record.

In this PR I propose to pass message descriptions to records with [@intl.description ...] annotation, ex:

let msg = [@intl.description "Description for message"] {id: "i.am.id", defaultMessage: "i.am.defaultMessage"};

In this case, only the extractor aware of message description, and we don't have to deal with {... description: None}, {... description: Some("description")} in the records, as we discussed before
cc @cknitt