docs: add message to remove default model for version 3.0 (#1563)

Author: shuoweil

README.rst

@@ -23,7 +23,7 @@ BigQuery DataFrames is an open-source package. You can run
             <li><strong>Remote Function Security:</strong>The library no longer automatically lets the Compute Engine default service account become the identity of the Cloud Run functions. If that is desired, it has to be indicated by passing cloud_function_service_account="default". And network ingress now defaults to "internal-only".</li>
             <li><strong>@remote_function Argument Passing:</strong> Arguments other than input_types, output_type, and dataset to remote_function must now be passed using keyword syntax, as positional arguments are no longer supported.</li>
             <li><strong>Endpoint Connections:</strong> Automatic fallback to locational endpoints in certain regions is removed. </li>
-            <li><strong>LLM Changes (Shift to Gemini):</strong> Integrations now default to <code>gemini-2.0-flash-001</code>. Support for PaLM2 models has been removed; migrate any PaLM2 usage to Gemini.</li>
+            <li><strong>LLM Updates (Gemini Integration):</strong> Integrations now default to the <code>gemini-2.0-flash-001</code> model. PaLM2 support has been removed; please migrate any existing PaLM2 usage to Gemini. Note: The current default model will be removed in Version 3.0.</li>
         </ul>
         <p><strong>Important:</strong> If you are not ready to adapt to these changes, please pin your dependency to a version less than 2.0 (e.g., <code>bigframes==1.42.0</code>) to avoid disruption.</p>
         <p style="margin-bottom: 0;"> To learn about these changes and how to migrate to version 2.0, see:  <a href="https://cloud.google.com/bigquery/docs/bigquery-dataframes-introduction" style="color: #842029; text-decoration: underline;">updated introduction guide</a>.</p>