Document the check_job_completion_time_unix log record attribute

Author: Vasileios Karakasis

docs/running.rst

@@ -862,6 +862,9 @@ All handlers accept the following set of attributes (keys) in their configuratio
     In this case, the accuracy depends on the execution policy used.
     If tests are executed with the serial execution policy, this is close to the real completion time, but if the asynchronous execution policy is used, it can differ significantly.
     If the job completion time cannot be retrieved, ``None`` will be printed.
+  - ``check_job_completion_time_unix``: *[new in 3.0]* The completion time of the job spawned by this regression test expressed as UNIX time.
+    This is a raw time field and will not be formatted according to ``datefmt``.
+    If specific formatting is desired, the ``check_job_completion_time`` should be used instead.
   - ``check_name``: Prints the name of the regression test on behalf of which ReFrame is currently executing.
     If ReFrame is not in the context of regression test, ``reframe`` will be printed.
   - ``check_num_tasks``: The number of tasks assigned to the regression test.