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.

---

📚 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 93.62%. Comparing base (b113fd5) to head (98d7e6e).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #5076   +/-   ##
========================================
  Coverage    93.62%   93.62%           
========================================
  Files          177      177           
  Lines        22001    22001           
  Branches      3114     3114           
========================================
  Hits         20599    20599           
  Misses         947      947           
  Partials       455      455           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.