docs(py-polars): clarify delta_write_options for compression

[cluster2600]

Closes #20739.

The delta_write_options parameter is forwarded to deltalake.write_deltalake, but compression is configured via deltalake.WriterProperties (passed as the writer_properties option).

This PR clarifies the docstring so users don't expect delta_write_options={"compression": ...} to work.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.39%. Comparing base (4929540) to head (91e1b50).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #26619      +/-   ##
==========================================
+ Coverage   81.37%   81.39%   +0.01%     
==========================================
  Files        1794     1794              
  Lines      244998   244998              
  Branches     3079     3079              
==========================================
+ Hits       199379   199425      +46     
+ Misses      44833    44787      -46     
  Partials      786      786              

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