[Storage] Add comment to clarify which package version is necessary (GoogleCloudPlatform#2315)

* Add comment to clarify which package version

* Lint and add another comment to related sample

Author: frankyn

storage/cloud-client/snippets.py

@@ -113,6 +113,8 @@ def list_buckets():
 def list_blobs(bucket_name):
     """Lists all the blobs in the bucket."""
     storage_client = storage.Client()
+
+    # Note: Client.list_blobs requires at least package version 1.17.0.
     blobs = storage_client.list_blobs(bucket_name)
 
     for blob in blobs:
@@ -142,6 +144,8 @@ def list_blobs_with_prefix(bucket_name, prefix, delimiter=None):
 
     """
     storage_client = storage.Client()
+
+    # Note: Client.list_blobs requires at least package version 1.17.0.
     blobs = storage_client.list_blobs(bucket_name, prefix=prefix,
                                       delimiter=delimiter)