docs: bigframes.options.bigquery.project and location are optional in some circumstances (#548)

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Follow-up to launch feedback and https://togithub.com/googleapis/python-bigquery-dataframes/pull/451
🦕

Author: tswast

samples/snippets/quickstart.py

@@ -29,8 +29,9 @@ def run_quickstart(project_id: str):
     import bigframes.pandas as bpd
 
     # Set BigQuery DataFrames options
+    # Note: The project option is not required in all environments.
+    # On BigQuery Studio, the project ID is automatically detected.
     bpd.options.bigquery.project = your_gcp_project_id
-    bpd.options.bigquery.location = "us"
 
     # Create a DataFrame from a BigQuery table
     query_or_table = "bigquery-public-data.ml_datasets.penguins"

samples/snippets/set_options_test.py

@@ -26,7 +26,14 @@ def test_bigquery_dataframes_set_options():
     REGION = "US"  # @param {type:"string"}
 
     # Set BigQuery DataFrames options
+    # Note: The project option is not required in all environments.
+    # On BigQuery Studio, the project ID is automatically detected.
     bpd.options.bigquery.project = PROJECT_ID
+
+    # Note: The location option is not required.
+    # It defaults to the location of the first table or query
+    # passed to read_gbq(). For APIs where a location can't be
+    # auto-detected, the location defaults to the "US" location.
     bpd.options.bigquery.location = REGION
 
     # [END bigquery_dataframes_set_options]