Update config_spec.md
Update documentation for `dataset_to_infer`
@@ -680,7 +680,8 @@ Interpolation order of the network outputs.
###### `dataset_to_infer`
String specifies which dataset ('all', 'training', 'validation', 'inference') to compute inference for.
-By default 'inference' dataset is used.
+By default 'inference' dataset is used. If no `dataset_split_file` is specified, then all data specified
+in the csv or search path are used for inference.
### EVALUATION
