Fixed #35130 -- Doc'd django.db.close_old_connections().

This also adds close_db_connections() to the django.db.__all__.

django/db/__init__.py

@@ -17,6 +17,7 @@ from django.db.utils import (
 from django.utils.connection import ConnectionProxy
 
 __all__ = [
+    "close_old_connections",
     "connection",
     "connections",
     "router",