docs(python): Adds cancellable queries

[thomasjpfan]

Closes #14544

This PR adds an example in the user guide about collect(background=True. The user guide also includes a warning stating that the feature may change at any point.

[coastalwhite]

as a non-breaking change

[thomasjpfan]

I can see how the phasing can be unclear. I updated this PR's wording to be the same as the docstring:

polars/py-polars/polars/lazyframe/frame.py

Lines 1882 to 1884 in ca6d46c

	.. warning::
	Background mode is considered **unstable**. It may be changed
	at any point without it being considered a breaking change.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.38%. Comparing base (55fc85a) to head (263f04a).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18041      +/-   ##
==========================================
- Coverage   80.46%   80.38%   -0.08%     
==========================================
  Files        1496     1496              
  Lines      197174   197480     +306     
  Branches     2820     2820              
==========================================
+ Hits       158655   158747      +92     
- Misses      37997    38211     +214     
  Partials      522      522              

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[thomasjpfan]

@coastalwhite Are you okay with the updated PR? It matches the wording from

polars/py-polars/polars/lazyframe/frame.py

Lines 1882 to 1884 in ca6d46c

	.. warning::
	Background mode is considered **unstable**. It may be changed
	at any point without it being considered a breaking change.