Update Documentation for win_scheduled_task.py (#66079)

Passing an empty string through this parameter is not gracefully handled by the module implementation, so adjust the documentation to only include the `PT0S` method of specifying unbounded `execution_time_limit`. The win_scheduled_task execution_time_limit must be set explicitly to an empty string to permit unbounded execution time.

A newly created task without a value for `TaskSettings.ExecutionTimeLimit` will default to 72 hours (https://docs.microsoft.com/en-us/windows/win32/taskschd/tasksettings-executiontimelimit) even if you omit the Ansible task parameter `execution_time_limit`.

lib/ansible/modules/windows/win_scheduled_task.py

@@ -353,7 +353,8 @@ options:
   execution_time_limit:
     description:
     - The amount of time allowed to complete the task.
-    - When not set, the time limit is infinite.
+    - When set to `PT0S`, the time limit is infinite.
+    - When omitted, the default time limit is 72 hours.
     - This is in the ISO 8601 Duration format C(P[n]Y[n]M[n]DT[n]H[n]M[n]S).
     type: str
     version_added: '2.5'