[doc] Document more of the existing Job properties

[ekouts]

Fixes #2215 .
I have added some description for completion_time and submit_time from the original issue. Should we add some documentation for other properties like name, workdir, max_pending_time, script_filename, stdout, stderr, sched_flex_alloc_nodes etc that currently don't appear in the reframe.core.schedulers.Job class?

[codecov-commenter]

Codecov Report

Merging #2227 (bdf1f6f) into master (8f37ff5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2227   +/-   ##
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  Coverage   85.54%   85.54%           
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  Files          55       55           
  Lines        9746     9746           
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  Hits         8337     8337           
  Misses       1409     1409           

Impacted Files	Coverage Δ
reframe/core/schedulers/__init__.py	98.36% <ø> (ø)

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[vkarak]

Lgtm. I don't think that we need to document the other properties that you mention, because these are also set at the test level, and it should be the preferred way to access them from there.