clarify past results

Author: jyu00

qiskit/fundamentals/3_the_ibmq_account.ipynb

@@ -1106,6 +1106,8 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
+    "Job Manager currently only supports new jobs, which means you cannot retrieve a `ManagedResults` instance for past jobs once the session is closed. You can, however, save the job IDs and use the traditional `backend.retrieve_job()` to retrive the jobs and their results.\n",
+    "\n",
     "You can use `job_set.jobs()` to retrieve a list of jobs within the job set. Note that an entry is `None` if that particular job could not be submitted."
    ]
   },