add pre_existing_label clarification

Author: pliablepixels

mlapiconfig.ini

@@ -330,6 +330,7 @@ ml_sequence= {
 		'face': {
 			'general':{
 				'pattern': '{{face_detection_pattern}}',
+				#'pre_existing_labels': ['person'], # when put in general section, it will check if a previous detection type (like object) found this label
 				'same_model_sequence_strategy': 'union' # combine results below
 			},
 			'sequence': [