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 80.85%. Comparing base (b4cde12) to head (8226ee8).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #24048      +/-   ##
==========================================
- Coverage   80.86%   80.85%   -0.01%     
==========================================
  Files        1101     1101              
  Lines      375444   375446       +2     
  Branches   375444   375446       +2     
==========================================
- Hits       303593   303578      -15     
- Misses      53764    53775      +11     
- Partials    18087    18093       +6     

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[2010YOUY01]

LGTM, this is a great doc improvement.

[2010YOUY01]

Suggested change

	/// Once known complete, the uffered rows are sorted by the full `(a, b, c)`
	/// Once known complete, the buffered rows are sorted by the full `(a, b, c)`

[NGA-TRAN]

Make sense. Thanks Andrew