docs: add details for bigquery_connection in @bpd.udf docstring (#1609)

* chore: add details for bigquery_connection in udf docstring

* quick fix

* Update bigframes/functions/_function_session.py

Co-authored-by: Tim Sweña (Swast) <swast@google.com>

* resolve the comments

* sync the docstring

---------

Co-authored-by: Tim Sweña (Swast) <swast@google.com>

Author: jialuoo

bigframes/functions/_function_session.py

@@ -775,6 +775,13 @@ def udf(
                 Name of the BigQuery connection. It is used to provide an
                 identity to the serverless instances running the user code. It
                 helps BigQuery manage and track the resources used by the udf.
+                This connection is required for internet access and for
+                interacting with other GCP services. To access GCP services, the
+                appropriate IAM permissions must also be granted to the
+                connection's Service Account. When it defaults to None, the udf
+                will be created without any connection. A udf without a
+                connection has no internet access and no access to other GCP
+                services.
             name (str, Optional):
                 Explicit name of the persisted BigQuery managed function. Use it
                 with caution, because more than one users working in the same

bigframes/session/__init__.py

@@ -1513,12 +1513,16 @@ def udf(
                 format. If this parameter is not provided then session dataset
                 id is used.
             bigquery_connection (str, Optional):
-                Name of the BigQuery connection. You should either have the
-                connection already created in the `location` you have chosen, or
-                you should have the Project IAM Admin role to enable the service
-                to create the connection for you if you need it. If this
-                parameter is not provided then the BigQuery connection from the
-                session is used.
+                Name of the BigQuery connection. It is used to provide an
+                identity to the serverless instances running the user code. It
+                helps BigQuery manage and track the resources used by the udf.
+                This connection is required for internet access and for
+                interacting with other GCP services. To access GCP services, the
+                appropriate IAM permissions must also be granted to the
+                connection's Service Account. When it defaults to None, the udf
+                will be created without any connection. A udf without a
+                connection has no internet access and no access to other GCP
+                services.
             name (str, Optional):
                 Explicit name of the persisted BigQuery managed function. Use it
                 with caution, because more than one users working in the same