Enhance the doc-string of CacheDataset

monai/data/dataset.py

@@ -503,6 +503,14 @@ class CacheDataset(Dataset):
     can be cached. During training, the dataset will load the cached results and run
     ``RandCropByPosNegLabeld`` and ``ToTensord``, as ``RandCropByPosNegLabeld`` is a randomized transform
     and the outcome not cached.
+
+    Note:
+        `CacheDataset` executes non-random transforms and prepares cache content in the main process before
+        the first epoch, then all the subprocesses of DataLoader will read the same cache content in the main process
+        during training. it may take a long time to prepare cache content according to the size of expected cache data.
+        So to debug or verify the program before real training, users can set `cache_rate=0.0` or `cache_num=0` to
+        temporarily skip caching.
+
     """
 
     def __init__(