Update all generated readme auth instructions [(googleapis#1121)](GoogleCloudPlatform/python-docs-samples#1121)

Change-Id: I03b5eaef8b17ac3dc3c0339fd2c7447bd3e11bd2

Author: Jon Wayne Parrott

samples/samples/README.rst

@@ -17,34 +17,12 @@ Setup
 Authentication
 ++++++++++++++
 
-Authentication is typically done through `Application Default Credentials`_,
-which means you do not have to change the code to authenticate as long as
-your environment has credentials. You have a few options for setting up
-authentication:
+This sample requires you to have authentication setup. Refer to the
+`Authentication Getting Started Guide`_ for instructions on setting up
+credentials for applications.
 
-#. When running locally, use the `Google Cloud SDK`_
-
-    .. code-block:: bash
-
-        gcloud auth application-default login
-
-
-#. When running on App Engine or Compute Engine, credentials are already
-   set-up. However, you may need to configure your Compute Engine instance
-   with `additional scopes`_.
-
-#. You can create a `Service Account key file`_. This file can be used to
-   authenticate to Google Cloud Platform services from any environment. To use
-   the file, set the ``GOOGLE_APPLICATION_CREDENTIALS`` environment variable to
-   the path to the key file, for example:
-
-    .. code-block:: bash
-
-        export GOOGLE_APPLICATION_CREDENTIALS=/path/to/service_account.json
-
-.. _Application Default Credentials: https://cloud.google.com/docs/authentication#getting_credentials_for_server-centric_flow
-.. _additional scopes: https://cloud.google.com/compute/docs/authentication#using
-.. _Service Account key file: https://developers.google.com/identity/protocols/OAuth2ServiceAccount#creatinganaccount
+.. _Authentication Getting Started Guide:
+    https://cloud.google.com/docs/authentication/getting-started
 
 Install Dependencies
 ++++++++++++++++++++
@@ -83,7 +61,7 @@ To run this sample:
 
     usage: snippets.py [-h] [--database-id DATABASE_ID]
                        instance_id
-                       {create_database,insert_data,query_data,read_data,add_column,update_data,query_data_with_new_column,read_write_transaction,read_only_transaction,add_index,query_data_with_index,read_data_with_index,add_storing_index,read_data_with_storing_index}
+                       {create_database,insert_data,query_data,read_data,read_stale_data,add_column,update_data,query_data_with_new_column,read_write_transaction,read_only_transaction,add_index,query_data_with_index,read_data_with_index,add_storing_index,read_data_with_storing_index}
                        ...
     
     This application demonstrates how to do basic operations using Cloud
@@ -93,13 +71,15 @@ To run this sample:
     
     positional arguments:
       instance_id           Your Cloud Spanner instance ID.
-      {create_database,insert_data,query_data,read_data,add_column,update_data,query_data_with_new_column,read_write_transaction,read_only_transaction,add_index,query_data_with_index,read_data_with_index,add_storing_index,read_data_with_storing_index}
+      {create_database,insert_data,query_data,read_data,read_stale_data,add_column,update_data,query_data_with_new_column,read_write_transaction,read_only_transaction,add_index,query_data_with_index,read_data_with_index,add_storing_index,read_data_with_storing_index}
         create_database     Creates a database and tables for sample data.
         insert_data         Inserts sample data into the given database. The
                             database and table must already exist and can be
                             created using `create_database`.
         query_data          Queries sample data from the database using SQL.
         read_data           Reads sample data from the database.
+        read_stale_data     Reads sample data from the database. The data is
+                            exactly 10 seconds stale.
         add_column          Adds a new column to the Albums table in the example
                             database.
         update_data         Updates sample data in the database. This updates the