fix: add description for transaction autocommit (#587)

Author: HemangChothani

packages/django-google-spanner/django_spanner/base.py

@@ -199,6 +199,10 @@ def is_usable(self):
 
         return True
 
+    # The usual way to start a transaction is to turn autocommit off.
+    # Spanner DB API does not properly start a transaction when disabling
+    # autocommit. To avoid this buggy behavior and to actually enter a new
+    # transaction, an explicit SELECT 1 is required.
     def _start_transaction_under_autocommit(self):
         """
         Start a transaction explicitly in autocommit mode.