docs: Add more documentation about PartialSortExec operator

[alamb]

Which issue does this PR close?

N/A

Rationale for this change

I was trying to explain to someone recently how PartialSortExec works, and I found the
datafusion docs weren't quite as explicit about the befits as I wanted

What changes are included in this PR?

1. Update the PartialSortExec documentation to explain the streaming and memory implications better, and add a worked buffering example

Are these changes tested?

By CI

Are there any user-facing changes?

Better docs

[alamb]

FYI @NGA-TRAN per our disucssion

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.87%. Comparing base (f9dde71) to head (2d3fa42).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24048      +/-   ##
==========================================
- Coverage   80.88%   80.87%   -0.01%     
==========================================
  Files        1102     1102              
  Lines      375813   375813              
  Branches   375813   375813              
==========================================
- Hits       303966   303953      -13     
- Misses      53745    53752       +7     
- Partials    18102    18108       +6     

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[2010YOUY01]

LGTM, this is a great doc improvement.

[NGA-TRAN]

Make sense. Thanks Andrew

[alamb]

Thank you for the review @2010YOUY01 and @NGA-TRAN