Comments for more allocs for custom ODE solvers

[DanielDoehring]

In #2078 I added some comments in the tests why for some examples there are more allocations permitted than for others. To avoid confusion, I added comments in the relevant places.

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.32%. Comparing base (ef78716) to head (27375f9).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2079   +/-   ##
=======================================
  Coverage   96.32%   96.32%           
=======================================
  Files         470      470           
  Lines       37485    37485           
=======================================
  Hits        36106    36106           
  Misses       1379     1379           

Flag	Coverage Δ
unittests	96.32% <ø> (ø)

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[bennibolm]

Good idea to add those comments. Thanks.

[ranocha]

Thanks