docs: driver-api: virtio: parenthesize external reference targets

Parenthesize targets to links in "References" section to distinguish
them from remaining texts.

While at it, describe the second target.

Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Message-Id: <20221220095828.27588-2-bagasdotme@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>