Docstrings moved to class-level for Neptune Objects

[Raalsky]

Before submitting checklist

• Did you update the CHANGELOG? (not for test updates, internal changes/refactors or CI/CD setup)
• Did you ask the docs owner to review all the user-facing changes?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (0a3bad5) 79.99% compared to head (580622b) 76.45%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1580      +/-   ##
==========================================
- Coverage   79.99%   76.45%   -3.54%     
==========================================
  Files         292      292              
  Lines       14540    14540              
==========================================
- Hits        11631    11117     -514     
- Misses       2909     3423     +514     

Flag	Coverage Δ
e2e-management	?
e2e-s3	?
e2e-s3-gcs	?
e2e-standard	?
macos	73.51% <ø> (-6.23%)	⬇️
py3.7	73.80% <ø> (-5.66%)	⬇️
py3.9	?
ubuntu	73.66% <ø> (-6.03%)	⬇️
unit	73.80% <ø> (-0.55%)	⬇️
windows	72.98% <ø> (-6.72%)	⬇️

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