fix(python): Fix return type for add_business_days, millennium, century and combine methods in Series.dt namespace

[Terrigible]

Return type for Series.dt.add_business_days's type annotation was fixed in #20402, but not its docstring. This PR fixes the docstring.

This PR also fixes the return types for millennium, century and combine in the Series.dt namespace.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 58.73%. Comparing base (93ceacc) to head (59b2994).
Report is 3 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main   #20436       +/-   ##
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- Coverage   79.01%   58.73%   -20.28%     
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  Files        1562     1562               
  Lines      220177   220177               
  Branches     2488     2488               
===========================================
- Hits       173980   129331    -44649     
- Misses      45624    90273    +44649     
  Partials      573      573               

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[MarcoGorelli]

thanks @Terrigible , much appreciated!