chore: add comment to cloudsql unix socket sample (GoogleCloudPlatform#7219)

* chore: add comment to unix socket sample

Author: jackwotherspoon

cloud-sql/postgres/sqlalchemy/main.py

@@ -164,6 +164,8 @@ def init_unix_connection_engine(db_config):
         # Equivalent URL:
         # postgresql+pg8000://:@/
         #                         ?unix_sock=//.s.PGSQL.5432
+        # Note: Some drivers require the `unix_sock` query parameter to use a different key.
+        # For example, 'psycopg2' uses the path set to `host` in order to connect successfully.
         sqlalchemy.engine.url.URL.create(
             drivername="postgresql+pg8000",
             username=db_user,  # e.g. "my-database-user"