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",

docs/ref/databases.txt

@@ -99,7 +99,8 @@ connections.
 
 If a connection is created in a long-running process, outside of Django’s
 request-response cycle, the connection will remain open until explicitly
-closed, or timeout occurs.
+closed, or timeout occurs. You can use ``django.db.close_old_connections()`` to
+close all old or unusable connections.
 
 Encoding
 --------