chore(arns): add lua type docs and update tests for arns.lua

[dtfiedler]

A series of changes will come to add docs to functions and standardize our error logic

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.00000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 89.93%. Comparing base (fcef672) to head (0a748b0).

Files with missing lines	Patch %	Lines
src/arns.lua	96.29%	2 Missing ⚠️
src/demand.lua	50.00%	2 Missing ⚠️
src/epochs.lua	33.33%	2 Missing ⚠️
src/utils.lua	75.00%	1 Missing ⚠️

Additional details and impacted files

@@                      Coverage Diff                      @@
##           PE-5712_allowed_delegates     #162      +/-   ##
=============================================================
+ Coverage                      89.58%   89.93%   +0.35%     
=============================================================
  Files                              9        9              
  Lines                           2045     2027      -18     
=============================================================
- Hits                            1832     1823       -9     
+ Misses                           213      204       -9     

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