docs: address sample feedback issues (GoogleCloudPlatform#5329)

## Description

Fixes GoogleCloudPlatform#5180, captures work from GoogleCloudPlatform#5181 authored by @keegan2149, thank you!

## Checklist
- [x] I have followed [Sample Guidelines from AUTHORING_GUIDE.MD](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md)
- [x] README is updated to include [all relevant information](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#readme-file)
- [x] **Tests** pass:   `nox -s py-3.6` (see [Test Environment Setup](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#test-environment-setup))
- [x] **Lint** pass:   `nox -s lint` (see [Test Environment Setup](https://github.com/GoogleCloudPlatform/python-docs-samples/blob/master/AUTHORING_GUIDE.md#test-environment-setup))
- [x] Please **merge** this PR for me once it is approved.

Author: frankyn

storage/cloud-client/snippets_test.py

@@ -405,15 +405,13 @@ def test_object_get_kms_key(test_bucket):
 
 def test_storage_compose_file(test_bucket):
     source_files = ["test_upload_blob_1", "test_upload_blob_2"]
-    blob_list = []
     for source in source_files:
         blob = test_bucket.blob(source)
         blob.upload_from_string(source)
-        blob_list.append(blob)
 
     with tempfile.NamedTemporaryFile() as dest_file:
         destination = storage_compose_file.compose_file(
-            test_bucket.name, blob_list, dest_file.name
+            test_bucket.name, source_files[0], source_files[1], dest_file.name
         )
         composed = destination.download_as_string()
 

storage/cloud-client/storage_change_file_storage_class.py

@@ -18,7 +18,6 @@
 
 # [START storage_change_file_storage_class]
 from google.cloud import storage
-from google.cloud.storage import constants
 
 
 def change_file_storage_class(bucket_name, blob_name):
@@ -30,7 +29,7 @@ def change_file_storage_class(bucket_name, blob_name):
 
     bucket = storage_client.get_bucket(bucket_name)
     blob = bucket.get_blob(blob_name)
-    blob.update_storage_class(constants.NEARLINE_STORAGE_CLASS)
+    blob.update_storage_class("NEARLINE")
 
     print(
         "Blob {} in bucket {} had its storage class set to {}".format(

storage/cloud-client/storage_compose_file.py

@@ -20,21 +20,25 @@
 from google.cloud import storage
 
 
-def compose_file(bucket_name, sources, destination_blob_name):
+def compose_file(bucket_name, first_blob_name, second_blob_name, destination_blob_name):
     """Concatenate source blobs into destination blob."""
     # bucket_name = "your-bucket-name"
-    # sources = [blob_1, blob_2]
+    # first_blob_name = "first-object-name"
+    # second_blob_name = "second-blob-name"
     # destination_blob_name = "destination-object-name"
 
     storage_client = storage.Client()
     bucket = storage_client.bucket(bucket_name)
     destination = bucket.blob(destination_blob_name)
     destination.content_type = "text/plain"
+
+    # sources is a list of Blob instances, up to the max of 32 instances per request
+    sources = [bucket.get_blob(first_blob_name), bucket.get_blob(second_blob_name)]
     destination.compose(sources)
 
     print(
-        "Composed new object {} in the bucket {}".format(
-            destination_blob_name, bucket.name
+        "New composite object {} in the bucket {} was created by combining {} and {}".format(
+            destination_blob_name, bucket_name, first_blob_name, second_blob_name
         )
     )
     return destination
@@ -44,5 +48,8 @@ def compose_file(bucket_name, sources, destination_blob_name):
 
 if __name__ == "__main__":
     compose_file(
-        bucket_name=sys.argv[1], sources=sys.argv[2], destination_blob_name=sys.argv[3],
+        bucket_name=sys.argv[1],
+        first_blob_name=sys.argv[2],
+        second_blob_name=sys.argv[3],
+        destination_blob_name=sys.argv[4],
     )