Add Documentation DO/DON'T section in agents.md

[suri-kumkaran]

Adds a Documentation DO/DON'T section to agents.md alongside the existing Test Patterns block.

Surfaced during review of #1027 and other PRs, where module- and item-level docs drifted toward manually-maintained type listings and signature restatements. New section captures the "less is more" framing, reminds agents not to duplicate what rustdoc already provides, and codifies the three patterns the reviewer called out as noise.

[Copilot]

Pull request overview

Adds contributor guidance to keep Rust documentation focused on durable semantics rather than duplicating rustdoc output, helping prevent module/item docs from drifting into manually maintained type/signature listings.

Changes:

• Adds a new “Documentation” DO/DON’T section to agents.md.
• Encourages using cargo doc --no-deps to rely on rustdoc-generated structure (signatures, listings, re-exports, links).
• Codifies specific anti-patterns to avoid (manual type/function lists, signature restatements, unnecessary docs for internal visibility items).

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.45%. Comparing base (e3139b4) to head (9e00a09).
⚠️ Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1103      +/-   ##
==========================================
- Coverage   89.49%   89.45%   -0.04%     
==========================================
  Files         479      484       +5     
  Lines       90337    91407    +1070     
==========================================
+ Hits        80844    81765     +921     
- Misses       9493     9642     +149     

Flag	Coverage Δ
miri	89.45% <ø> (-0.04%)	⬇️
unittests	89.10% <ø> (-0.04%)	⬇️

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see 84 files with indirect coverage changes

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