fix AnalysisBase docs

[orbeckst]

Fixes #5075

Changes made in this Pull Request:

• document attributes n_frames, start, stop, step

PR Checklist

• Issue raised/referenced?
• Tests updated/added?
• Documentation updated/added?
• package/CHANGELOG file updated?
• Is your name in package/AUTHORS? (If it is not, add it!)

Developers Certificate of Origin

I certify that I can submit this code contribution as described in the Developer Certificate of Origin, under the MDAnalysis LICENSE.

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📚 Documentation preview 📚: https://mdanalysis--5076.org.readthedocs.build/en/5076/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.63%. Comparing base (b113fd5) to head (ce14e6e).
Report is 2 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #5076   +/-   ##
========================================
  Coverage    93.62%   93.63%           
========================================
  Files          177      177           
  Lines        22001    22033   +32     
  Branches      3114     3115    +1     
========================================
+ Hits         20599    20631   +32     
- Misses         947      948    +1     
+ Partials       455      454    -1     

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[orbeckst]

Thanks for looking at the PR & suggestions, which I merged.

Do you mind me being PR shepherd?