drm/xe/uapi: Document use of size in drm_xe_device_query

Document the behavior of the driver for IOCTL DRM_IOCTL_XE_DEVICE_QUERY
depending on the size value provided in struct drm_xe_device_query.
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Acked-by: José Roberto de Souza <jose.souza@intel.com>
Acked-by: Mateusz Naklicki <mateusz.naklicki@intel.com>
Signed-off-by: Francois Dugast <francois.dugast@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

include/uapi/drm/xe_drm.h

@@ -467,9 +467,15 @@ struct drm_xe_query_topology_mask {
  * struct drm_xe_device_query - Input of &DRM_IOCTL_XE_DEVICE_QUERY - main
  * structure to query device information
  *
- * If size is set to 0, the driver fills it with the required size for the
- * requested type of data to query. If size is equal to the required size,
- * the queried information is copied into data.
+ * The user selects the type of data to query among DRM_XE_DEVICE_QUERY_*
+ * and sets the value in the query member. This determines the type of
+ * the structure provided by the driver in data, among struct drm_xe_query_*.
+ *
+ * If size is set to 0, the driver fills it with the required size for
+ * the requested type of data to query. If size is equal to the required
+ * size, the queried information is copied into data. If size is set to
+ * a value different from 0 and different from the required size, the
+ * IOCTL call returns -EINVAL.
  *
  * For example the following code snippet allows retrieving and printing
  * information about the device engines with DRM_XE_DEVICE_QUERY_ENGINES: