diff: update help output to match docs, et al.

[jorgeorpinel]

Followup to #3566 (review)

• ❗ I have followed the Contributing to DVC checklist.

• 📖 If this PR requires documentation updates, I have created a separate PR (or issue, at least) in dvc.org and linked it here. If the CLI API is changed, I have updated tab completion scripts.

• ❌ I will check DeepSource, CodeClimate, and other sanity checks below. (We consider them recommendatory and don't expect everything to be addressed. Please fix things that actually improve code or fix bugs.)

Thank you for the contribution - we'll try to review it as soon as possible. 🙏

[codecov]

Codecov Report

Merging #3601 into master will decrease coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3601      +/-   ##
==========================================
- Coverage   90.91%   90.90%   -0.02%     
==========================================
  Files         143      143              
  Lines        9236     9236              
==========================================
- Hits         8397     8396       -1     
- Misses        839      840       +1     

Impacted Files	Coverage Δ
dvc/command/diff.py	92.98% <ø> (ø)
dvc/command/init.py	100.00% <ø> (ø)
dvc/scm/__init__.py	100.00% <ø> (ø)
dvc/scm/base.py	87.93% <ø> (ø)
dvc/output/base.py	95.45% <100.00%> (ø)
dvc/updater.py	54.65% <0.00%> (-1.17%)	⬇️

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[efiop]

Thanks @jorgeorpinel ! 🙏