docs: show array_distinct keeping first-seen order

[rawsun007]

Which issue does this PR close?

• None; documentation fix.

Rationale for this change

The array_distinct example says select array_distinct([1, 3, 2, 3, 1, 2, 4]) returns [1, 2, 3, 4]. It returns [1, 3, 2, 4]: #20364 changed it to keep elements in the order they first appear, matching array_union and array_intersect, and the example was not updated.

What changes are included in this PR?

The #[user_doc] example and description in set_ops.rs, the matching code comment, and scalar_functions.md regenerated with ./dev/update_function_docs.sh. Also a sqllogictest case for the order, since every existing array_distinct case used input whose sorted and first-seen orders coincide.

What is the testing strategy for this PR?

Ran the example against datafusion-cli built from main. The new case in array_distinct.slt passes; cargo fmt --check, cargo clippy -p datafusion-functions-nested --all-targets -- -D warnings and ./ci/scripts/doc_prettier_check.sh pass.

Are there any user-facing changes?

Documentation and a test only.

Generated with Claude Code (claude-opus-5-5); I checked the output above.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.58%. Comparing base (54e32bb) to head (7182e84).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #25930      +/-   ##
==========================================
- Coverage   82.58%   82.58%   -0.01%     
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  Files        1144     1144              
  Lines      443198   443198              
  Branches   443198   443198              
==========================================
- Hits       366024   366013      -11     
- Misses      54978    54986       +8     
- Partials    22196    22199       +3     

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