docs: add bigframes default connection warning

Author: GarrettWu

bigframes/core/blocks.py

@@ -442,7 +442,7 @@ def reset_index(
             level_ids = self.index_columns
 
         expr = self._expr
-        replacement_idx_type = replacement or self.session._default_index_type
+        replacement_idx_type = replacement or self.session._default_index_type  # type: ignore
         if set(self.index_columns) > set(level_ids):
             new_index_cols = [col for col in self.index_columns if col not in level_ids]
             new_index_labels = [self.col_id_to_index_name[id] for id in new_index_cols]
@@ -2382,7 +2382,7 @@ def merge(
         if (
             self.index.is_null
             or other.index.is_null
-            or self.session._default_index_type == bigframes.enums.DefaultIndexKind.NULL
+            or self.session._default_index_type == bigframes.enums.DefaultIndexKind.NULL  # type: ignore
         ):
             return Block(joined_expr, index_columns=[], column_labels=labels)
         elif index_cols:

bigframes/dataframe.py

@@ -4233,7 +4233,7 @@ def to_gbq(
 
             # The client code owns this table reference now
             temp_table_ref = (
-                self._session._anon_dataset_manager.generate_unique_resource_id()
+                self._session._anon_dataset_manager.generate_unique_resource_id()  # type: ignore
             )
             destination_table = f"{temp_table_ref.project}.{temp_table_ref.dataset_id}.{temp_table_ref.table_id}"
 

bigframes/session/__init__.py

@@ -374,6 +374,18 @@ def _allows_ambiguity(self) -> bool:
     def _anonymous_dataset(self):
         return self._anon_dataset_manager.dataset
 
+    @property
+    def _bq_connection(self) -> str:
+        msg = bfe.format_message(
+            f"You are using the BigFrames session default connection: {self._bq_connection}, which can be different from the BigQuery project default connection."
+        )
+        warnings.warn(msg, category=FutureWarning)
+        return self._bq_connection
+
+    @_bq_connection.setter
+    def _bq_connection(self, value):
+        self._bq_connection = value
+
     def __hash__(self):
         # Stable hash needed to use in expression tree
         return hash(str(self._session_id))