Clarify the logger message of BirchClustering

[JiQi535]

Summary

Clarify the logger message of BirchClustering. When n is set not to an integer instead of None, the number of clusters returned by BIRCH is actually equal to n, instead of the number of clusters dependent on threshold. This is discussed in details on the documentation of sklearn for BIRCH, when n_clusters is an integer or None.

Checklist

• Google format doc strings added. Check with ruff.
• Type annotations included. Check with mypy.
• Tests added for new features/fixes.
• If applicable, new classes/functions/modules have duecredit @due.dcite decorators to reference relevant papers by DOI (example)

Tip: Install pre-commit hooks to auto-check types and linting before every commit:

pip install -U pre-commit
pre-commit install

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.58%. Comparing base (2c1bf6a) to head (56a5505).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #604      +/-   ##
==========================================
- Coverage   67.64%   67.58%   -0.06%     
==========================================
  Files          68       68              
  Lines        5263     5263              
==========================================
- Hits         3560     3557       -3     
- Misses       1703     1706       +3     

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