Default seed_everything(workers=True) in the LightningCLI

[carmocca]

What does this PR do?

See title, also other minor changes:

• Includes some docs improvements
• Removes duplicate print statements (running trainer.test already prints the result)
• Adds missing "#"s to the CHANGELOG

Before submitting

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• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

PR review

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[codecov]

Codecov Report

Merging #7504 (170ef58) into master (83283fd) will decrease coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #7504    +/-   ##
=======================================
- Coverage      92%     88%    -4%     
=======================================
  Files         199     199            
  Lines       13068   13068            
=======================================
- Hits        12026   11438   -588     
- Misses       1042    1630   +588