Returns the subject the service would put on the email, so a UI can seed an editable Subject
field from the same resolution the send uses rather than reimplementing it.

Give documentId for a document that exists. The answer is then exactly what a send with no
subject override produces: a token the service holds no value for resolves to an empty
string, as it would in the delivered email.

Give documentKind, sourceKind and sourceId instead for a document about to be created.
A token whose value is only assigned at creation is then returned verbatim, for example
Grand Hotel - Invoice #[Invoice Number]. Send the string back, edited or not, on the create
request and the service fills those tokens in once the values exist.

Supplying the chosen recipient lets the guest's language win when the property has
"use guest language" enabled; without it the subject falls back to the property's language.