docs(python): Document write_ipc buffer behavior with file=None

[uurl]

Closes #18200.

Documents the behavior of DataFrame.write_ipc(file=None).

When file=None, write_ipc returns a BytesIO object. The buffer position is left at the end of the written data, so users need to call seek(0) before reading it back with pl.read_ipc.

Added a short docstring example showing:

buf = df.write_ipc(file=None)
buf.seek(0)
pl.read_ipc(buf).equals(df)

AI usage:

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• I confirm that I have reviewed all changes myself, and I believe they are relevant and correct.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.29%. Comparing base (693c520) to head (3299251).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #27430      +/-   ##
==========================================
+ Coverage   81.19%   81.29%   +0.10%     
==========================================
  Files        1832     1832              
  Lines      253247   253263      +16     
  Branches     3176     3176              
==========================================
+ Hits       205626   205895     +269     
+ Misses      46798    46545     -253     
  Partials      823      823              

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