drm: Introduce documentation for hotspot properties

To clarify the intent and reasoning behind the hotspot properties
introduce userspace documentation that goes over cursor handling
in para-virtualized environments.

The documentation is generic enough to not special case for any
specific hypervisor and should apply equally to all.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Michael Banack <banackm@vmware.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231023074613.41327-10-aesteve@redhat.com