docs: mention output_rows_skew on partitioned operators in explain-usage

[LiaCastaneda]

Follow-up to #25977.

Which issue does this PR close?

• Closes #.

Rationale for this change

now output_rows_skew is used in other operators than DataSourceExec, but I forgot to add it to the doc

What changes are included in this PR?

Small comment in the doc mentioning output_rows_skew is also used for other operators than DataSourceExec(parquet)

What is the testing strategy for this PR?

Are there any user-facing changes?

Just doc.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82.66%. Comparing base (c3ef346) to head (cb00152).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #26048   +/-   ##
=======================================
  Coverage   82.66%   82.66%           
=======================================
  Files        1147     1147           
  Lines      446357   446430   +73     
  Branches   446357   446430   +73     
=======================================
+ Hits       368971   369032   +61     
+ Misses      54997    54996    -1     
- Partials    22389    22402   +13     

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