Share a company document with other companies

Gives the listed companies access to a document, approved by the caller, and notifies each of them. The body is a bare array of company ids. Anyone who can see the document may share it — not only its owner. The document's owner, the company it is filed on, ids naming no company, and companies that already hold an approved share are skipped; a pending access request from a listed company is approved in place. Responds with the companies this call newly gave access to, which can be none. A document the caller cannot see answers 404; a public one answers 409 company_document_already_public. Notifications are best-effort: access is granted even if one cannot be published.