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, points at cargo doc --no-deps, 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 89.49%. Comparing base (e3139b4) to head (1f7d7e1).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1103   +/-   ##
=======================================
  Coverage   89.49%   89.49%           
=======================================
  Files         479      479           
  Lines       90337    90337           
=======================================
+ Hits        80844    80847    +3     
+ Misses       9493     9490    -3     

Flag	Coverage Δ
miri	89.49% <ø> (+<0.01%)	⬆️
unittests	89.14% <ø> (+<0.01%)	⬆️

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see 1 file with indirect coverage changes

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[hildebrandmw]

Not sure this is advice we want to be giving agents as it introduces another thing to build, output to read, and potentially bloats context. Perhaps reword as "don't duplicate what rustdoc already does".