virtio: standardize Returns documentation style

virtio: standardize Returns documentation style

Remove colons after "Returns" in virtio_map_ops function
documentation - both to avoid triggering an htmldoc warning
and for consistency with virtio_config_ops.

This affects map_page, alloc, need_sync, and max_mapping_size.

Fixes: bee8c7c24b73 ("virtio: introduce map ops in virtio core")
Message-Id: <c262893fa21f4b1265147ef864574a9bd173348f.1763026134.git.mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>