qapi/rocker: Fix up doc comment notes on optional members

qapi/rocker: Fix up doc comment notes on optional members

Talking about #optional like this

    # Note: fields are marked #optional to indicate that they may or may
    # not appear ...

doesn't work so well in generated documentation, because the #optional
tag is not visible there.  Replace by

    # Note: optional members may or may not appear ...

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <1489582656-31133-13-git-send-email-armbru@redhat.com>