fix StructuredMesh docstrings

[ranocha]

Some docstrings were missing the periodicity argument.

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.35%. Comparing base (3716df4) to head (1da044c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2208   +/-   ##
=======================================
  Coverage   96.35%   96.35%           
=======================================
  Files         485      485           
  Lines       38974    38974           
=======================================
  Hits        37553    37553           
  Misses       1421     1421           

Flag	Coverage Δ
unittests	96.35% <100.00%> (ø)

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