vringh: address kdoc warnings

Address some minor kdoc warnings in vring.h.
* Place kdoc for 'struct vringh_config_ops' immediately before the structure
* Add missing documentation of members of 'vringh_iov' and 'vringh_kiov'

Warnings flagged by:
 $ ./scripts/kernel-doc -none include/linux/vringh.h
 include/linux/vringh.h:68: error: Cannot parse struct or union!
 include/linux/vringh.h:92: warning: Function parameter or member 'iov' not described in 'vringh_iov'
 include/linux/vringh.h:92: warning: Function parameter or member 'consumed' not described in 'vringh_iov'
 include/linux/vringh.h:92: warning: Function parameter or member 'i' not described in 'vringh_iov'
 include/linux/vringh.h:92: warning: Function parameter or member 'used' not described in 'vringh_iov'
 include/linux/vringh.h:92: warning: Function parameter or member 'max_num' not described in 'vringh_iov'
 include/linux/vringh.h:104: warning: Function parameter or member 'iov' not described in 'vringh_kiov'
 include/linux/vringh.h:104: warning: Function parameter or member 'consumed' not described in 'vringh_kiov'
 include/linux/vringh.h:104: warning: Function parameter or member 'i' not described in 'vringh_kiov'
 include/linux/vringh.h:104: warning: Function parameter or member 'used' not described in 'vringh_kiov'
 include/linux/vringh.h:104: warning: Function parameter or member 'max_num' not described in 'vringh_kiov'

Signed-off-by: Simon Horman <horms@kernel.org>
Message-Id: <20230331-vhost-fixes-v1-2-1f046e735b9e@kernel.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>