[AIRFLOW-XXXX] Clear debug docs

[potiuk]

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Issue link: Document only change, no JIRA issue

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[potiuk]

@nuclearpinguin @ashb @kaxil @mik-laj @xinbinhuang - What do you think about adding a bit more documentation on console debugging?

With Python 3.7, pudb, web-pdb it's so much nicer and easier https://hackernoon.com/python-3-7s-new-builtin-breakpoint-a-quick-tour-4f1aebc444c

[mik-laj]

I think it's worth describing the --post_mortem parameter also. For now, I want to add support for the other debuggers, but...unfortunately, I do not have time to write a guide describing in detail the use.
#7105

usage: airflow tasks test [-h] [-sd SUBDIR] [-dr] [-tp TASK_PARAMS] [-pm]
                          dag_id task_id execution_date
[...]
  -pm, --post_mortem    Open debugger on uncaught exception