[Job submission] Improve job submission docs (#23115)

I am following job submission docs here https://docs.ray.io/en/latest/cluster/job-submission.html  and run some examples.

I notice there're few minor issues.

1. some required libraries are not imported in any code snippets
2.  Get job api returns `{'status': 'SUCCEEDED'}` instead of `job_status` so code snippet here doesn't work https://docs.ray.io/en/latest/cluster/job-submission.html#rest-api

doc/source/cluster/job-submission.rst

@@ -203,6 +203,7 @@ Now we can have a simple polling loop that checks the job status until it reache
 .. code-block:: python
 
     from ray.job_submission import JobStatus
+    import time
 
     def wait_until_finish(job_id):
         start = time.time()
@@ -264,6 +265,10 @@ Under the hood, both the Job Client and the CLI make HTTP calls to the job serve
 
 .. code-block:: python
 
+    import requests
+    import json
+    import time
+
     resp = requests.post(
         "http://127.0.0.1:8265/api/jobs/",
         json={
@@ -286,7 +291,7 @@ Under the hood, both the Job Client and the CLI make HTTP calls to the job serve
             "http://127.0.0.1:8265/api/jobs/<job_id>"
         )
         rst = json.loads(resp.text)
-        status = rst["job_status"]
+        status = rst["status"]
         print(f"status: {status}")
         if status in {JobStatus.SUCCEEDED, JobStatus.STOPPED, JobStatus.FAILED}:
             break