docs: clarify ADC pre-auth in a non-interactive environment (#348)

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Fixes internal issue 321859738 🦕

Author: shobsi

README.rst

@@ -27,10 +27,13 @@ Prerequisites
 
 * Install the ``bigframes`` package.
 * Create a Google Cloud project and billing account.
-* When running locally, authenticate with application default credentials. See
-  the `gcloud auth application-default login
-  <https://cloud.google.com/sdk/gcloud/reference/auth/application-default/login>`_
-  reference.
+* In an interactive environment (like Notebook, Python REPL or command line),
+  ``bigframes`` will do the authentication on-the-fly if needed. Otherwise, see
+  `how to set up application default credentials <https://cloud.google.com/docs/authentication/provide-credentials-adc>`_
+  for various environments. For example, to pre-authenticate on your laptop you can
+  `install and initialize the gcloud CLI <https://cloud.google.com/sdk/docs/install>`_,
+  and then generate the application default credentials by doing
+  `gcloud auth application-default login <https://cloud.google.com/sdk/gcloud/reference/auth/application-default/login>`_.
 
 Code sample
 ^^^^^^^^^^^
@@ -286,9 +289,6 @@ created by BigQuery DataFrames are prefixed by ``bigframes``.
 
 **Requirements**
 
-BigQuery DataFrames uses the ``gcloud`` command-line interface internally,
-so you must run ``gcloud auth login`` before using remote functions.
-
 To use BigQuery DataFrames remote functions, you must enable the following APIs:
 
 * The BigQuery API (bigquery.googleapis.com)