docs: indicate that project and location are optional in example notebooks (#451)

Author: tswast

notebooks/dataframes/dataframe.ipynb

@@ -371,7 +371,14 @@
       },
       "outputs": [],
       "source": [
+        "# Note: The project option is not required in all environments.\n",
+        "# On BigQuery Studio, the project ID is automatically detected.\n",
         "bpd.options.bigquery.project = PROJECT_ID\n",
+        "\n",
+        "# Note: The location option is not required.\n",
+        "# It defaults to the location of the first table or query\n",
+        "# passed to read_gbq(). For APIs where a location can't be\n",
+        "# auto-detected, the location defaults to the \"US\" location.\n",
         "bpd.options.bigquery.location = LOCATION"
       ]
     },