CLI documentation improvements

[Thomsch]

Description

The command line subcommands do not have descriptions that make it hard to use Oríon without looking at the documentation.

Changes

This PR fixes a few documentation problems and adds a short and long description for each subcommand, indicating what are their purposes.

Checklist

Tests

• I added corresponding tests for bug fixes and new features. If possible, the tests fail without the changes
• All new and existing tests are passing ($ tox -e py38; replace 38 by your Python version if necessary)

Documentation

• I have updated the relevant documentation related to my changes

Quality

• I have read the CONTRIBUTING doc
• My commits messages follow this format
• My code follows the style guidelines ($ tox -e lint)

[codecov-commenter]

Codecov Report

Merging #440 into develop will increase coverage by 0.00%.
The diff coverage is 50.00%.

@@           Coverage Diff            @@
##           develop     #440   +/-   ##
========================================
  Coverage    48.95%   48.95%           
========================================
  Files           80       80           
  Lines        13640    13641    +1     
  Branches       329      329           
========================================
+ Hits          6677     6678    +1     
  Misses        6946     6946           
  Partials        17       17           

Impacted Files	Coverage Δ
src/orion/core/cli/test_db.py	57.14% <50.00%> (+3.29%)	⬆️

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[bouthilx]

Thanks!! :)