vdpa: address kdoc warnings

This patch addresses the following minor kdoc problems.

* Incorrect spelling of 'callback' and 'notification'
* Unrecognised kdoc format for 'struct vdpa_map_file'
* Missing documentation of 'get_vendor_vq_stats' member of
  'struct vdpa_config_ops'
* Missing documentation of 'max_supported_vqs' and 'supported_features'
  members of 'struct vdpa_mgmt_dev'

Most of these problems were flagged by:

 $ ./scripts/kernel-doc -Werror -none  include/linux/vdpa.h
 include/linux/vdpa.h:20: warning: expecting prototype for struct vdpa_calllback. Prototype was for struct vdpa_callback instead
 include/linux/vdpa.h:117: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
  * Corresponding file area for device memory mapping
 include/linux/vdpa.h:357: warning: Function parameter or member 'get_vendor_vq_stats' not described in 'vdpa_config_ops'
 include/linux/vdpa.h:518: warning: Function parameter or member 'supported_features' not described in 'vdpa_mgmt_dev'
 include/linux/vdpa.h:518: warning: Function parameter or member 'max_supported_vqs' not described in 'vdpa_mgmt_dev'

The misspelling of 'notification' was flagged by:
 $ ./scripts/checkpatch.pl --codespell --showfile --strict -f include/linux/vdpa.h
 include/linux/vdpa.h:171: CHECK: 'notifcation' may be misspelled - perhaps 'notification'?
 ...

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