test: check docstrings

[gdalle]

Tests

• In formalities tests for DI and DIT, check that every public name has a docstring (see Test if all exported names has a docstring JuliaTesting/Aqua.jl#90).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.53%. Comparing base (6b55559) to head (b8a90ee).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #640   +/-   ##
=======================================
  Coverage   97.53%   97.53%           
=======================================
  Files         111      111           
  Lines        5551     5551           
=======================================
  Hits         5414     5414           
  Misses        137      137           

Flag	Coverage Δ
DI	98.65% <ø> (ø)
DIT	95.35% <ø> (ø)

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