docs: Clarify assisted contribution expectations

[tomatotomata]

Summary

• add a short section for contributors using code or writing assistants
• ask contributors to disclose assistance and review the rendered diff and description
• reinforce focused, actionable review communication

Verification

• git diff --check
• documentation-only change

Assistance disclosure

I used an LLM to help draft the wording, then reviewed and edited the final text and verified the rendered diff manually.

Relates to: #3939

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.54%. Comparing base (e496a87) to head (fbe54ce).

Additional details and impacted files

@@ Coverage Diff @@## master #4427 +/- ##
=======================================
Coverage 97.54% 97.54% =======================================
Files 194 194 Lines 19836 19836 =======================================
Hits 19350 19350 + Misses 269 268 -1 - Partials 217 218 +1 

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[gmlewis]

Although some of this is mentioned in the "Tips" section above it, I'm OK with adding this, but will NOT be closing the issue referred to in the description, as I have found that particular issue to be extremely useful to keep open.
LGTM.

cc: @stevehipwell - @alexandear - @Not-Dhananjay-Mishra