[DOCS] prepro image

Author: zsdonghao

docs/modules/prepro.rst

@@ -56,6 +56,7 @@ Threading
 Images
 -----------
 
+- These functions only apply on a single image, use ``threading_data`` to apply multiple threading see ``tutorial_image_preprocess.py``.
 - All functions have argument ``is_random``.
 - All functions end with `multi` , usually be used for image segmentation i.e. the input and output image should be matched.