docs(style): a docstring is not a changelog

[breimanntools]

The style guide said ".. versionchanged:: X.Y.Z when behaviour changes" and offered the parameter-level form only as an alternative. Agents followed it literally: a 2026-06-01 pass (a4537eed) backfilled version history into docstrings package-wide, and the API pages now open with a stack of "Changed in version 1.1.0" blocks before the reader learns what the method does.

Measured on origin/master: 242 versionadded + 40 versionchanged across the package, 11 of the latter in _cpp.py alone. CPP.__init__'s docstring went 36 → 102 → 117 lines across v1.0.3 → v1.1.0 → today, and the version stack is part of why.

The rule now

• exactly one .. versionadded:: per public symbol, marking when the symbol appeared
• never a .. versionchanged:: stack at class or method level
• a behaviour change is annotated on the parameter that changed — the scikit-learn placement
• everything else lives in CHANGELOG.md and the release notes, which is where a reader looking for history goes

Mirrored into .claude/rules/docstrings.md, so it reaches an agent before it writes the docstring, with the numbers included — the previous wording is exactly what produced them.

No library code touched and no existing docstring edited here. The cleanup of the 40 existing entries is tracked in #581, which carries the rendering target and the one decision it needs (whether the class-level versionadded is worth keeping at all).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.39%. Comparing base (3c11c90) to head (d93b612).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #587   +/-   ##
=======================================
  Coverage   95.39%   95.39%           
=======================================
  Files         222      222           
  Lines       23387    23387           
  Branches     4073     4073           
=======================================
  Hits        22309    22309           
  Misses        631      631           
  Partials      447      447           

Components	Coverage Δ
cpp_core	95.97% <ø> (ø)

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