DOC: Add docstring to parameter in base data module to_nifti function

[jhlegarreta]

Add docstring to parameter in base data module to_nifti function.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.04%. Comparing base (0ed2364) to head (805ffd6).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #359   +/-   ##
=======================================
  Coverage   80.04%   80.04%           
=======================================
  Files          34       34           
  Lines        1959     1959           
  Branches      207      207           
=======================================
  Hits         1568     1568           
  Misses        335      335           
  Partials       56       56           

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