docs: Document expectations of PartialReduce aggregate mode

[alamb]

Which issue does this PR close?

• Related to [EPIC] Split Aggregation Logic into Dedicated Streams #22710 and refactor(hash-aggr): Let partial-reduce mode aggregation error under memory limit #24486
• Follow on to feat: add AggregateMode::PartialReduce for tree-reduce aggregation #20019

Rationale for this change

When AggregateMode::PartialReduce was added in #20019 from @njsmith, I believe the intention was that it is purely a best-effort optimization: it reduces the volume of intermediate aggregate state (for example, before sending it over the network in a distributed plan), but it is not required to be fully reduced, and consumers must merge it regardless.

However, this contract was never explicitly written down, which came up while I was reviewing #24486 from @2010YOUY01

What changes are included in this PR?

Documentation only (no code changes): add a "Best-Effort Reduction" section to
the AggregateMode::PartialReduce variant docs stating the output contract.

Are these changes tested?

Covered by CI docs checks (cargo doc passes with -D warnings).

Are there any user-facing changes?

Documentation only.

[alamb]

@njsmith and @gabotechs does this match your understanding of what PartialReduce is supposed to do?

[gabotechs]

It does yeah

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.43%. Comparing base (26b40dd) to head (f541634).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24640      +/-   ##
==========================================
- Coverage   81.43%   81.43%   -0.01%     
==========================================
  Files        1118     1118              
  Lines      399414   399414              
  Branches   399414   399414              
==========================================
- Hits       325278   325275       -3     
  Misses      55145    55145              
- Partials    18991    18994       +3     

☔ View full report in Codecov by Harness.
📢 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.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[gabotechs]

This makes sense, thanks @alamb! would love to hear if @njsmith had other non-best-effort use case in mind.