docs: Clarify DataFrame.sort returns new frame

[uurl]

Fixes #17636.

Clarifies the DataFrame.sort docstring to state that it returns a new sorted DataFrame, rather than implying in-place mutation.

• I used AI to help identify the affected docstring and draft the PR description.
• I confirm that I have reviewed the change myself, and I believe it is relevant and correct.

[orlp]

Hi, I've noticed a lot of small docs PRs coming in. I appreciate the effort but I'd suggest sticking to issues which have the Accepted label to reduce wasted (review) effort.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.14%. Comparing base (005b111) to head (5de058e).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #27442      +/-   ##
==========================================
- Coverage   81.62%   80.14%   -1.48%     
==========================================
  Files        1836     1836              
  Lines      254252   254252              
  Branches     3180     3180              
==========================================
- Hits       207527   203767    -3760     
- Misses      45901    49661    +3760     
  Partials      824      824              

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