message: Document possible values of the nnapi_execution_preference option
diff: @@ -68,7 +68,8 @@ where applicable. For details about each parameter, please refer to
  * `use_nnapi`: `bool` (default=false) \
  Note some Android P devices will fail to use NNAPI for models in
  `/data/local/tmp/` and this benchmark tool will not correctly use NNAPI.
  -* `nnapi_execution_preference`: `str` (default="")
  +* `nnapi_execution_preference`: `str` (default="") \
  + Should be one of: `fast_single_answer`, `sustained_speed`, `low_power`, `undefined`
  * `nnapi_execution_priority`: `str` (default="") \
  Note this requires Android 11+.
  * `nnapi_accelerator_name`: `str` (default="") \
  