manage: Restore `changepassword` back to documented_commands.

The current help text in manage.py does not displays `changepassword`
command under the set of django commands subset. Hence, add back
`changepassword` to the help text.

manage.py

@@ -53,6 +53,7 @@ def get_filtered_commands() -> Dict[str, str]:
     ]
     documented_command_subsets = {
         "django.core": {
+            "changepassword",
             "dbshell",
             "makemigrations",
             "migrate",