docs(python): Clarify null handling in unique operations

[uurl]

Closes #7770.

Summary

Clarifies that null is considered a unique value in Python API documentation for unique operations.

Updated docs for:

• Series.unique
• Series.n_unique
• Expr.unique
• pl.n_unique

Also updates the pl.n_unique and Series.n_unique examples to include None.

Validation

git diff --check
ruff check py-polars/src/polars/series/series.py py-polars/src/polars/expr/expr.py py-polars/src/polars/functions/lazy.py
python -m pytest \
  py-polars/tests/unit/operations/unique/test_n_unique.py \
  py-polars/tests/unit/operations/unique/test_unique.py \
  -k "n_unique or unique"

• I used AI to help identify the narrow documentation scope and review the PR text.
• I confirm that I have reviewed all changes myself, and I believe they are relevant and correct.

[orlp]

This example now is missing a duplicate value. Can you make it 1, 1, None?

[uurl]

Good catch, updated the example to keep a duplicate value while including None. Thanks

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.55%. Comparing base (693c520) to head (2ae7877).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #27431      +/-   ##
==========================================
- Coverage   81.19%   79.55%   -1.64%     
==========================================
  Files        1832     1836       +4     
  Lines      253247   254204     +957     
  Branches     3176     3177       +1     
==========================================
- Hits       205626   202237    -3389     
- Misses      46798    51143    +4345     
- Partials      823      824       +1     

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