CAdES : rework CAdES signing API.
Make it private, as it is unused outside library bounds.
Fix varous doc-nits.
Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>