Document and clean sig on forward_diff_no_inf

[oxinabox]

I wanted to add a docstring as I had no idea what visit_custom! did.
And then i noticed that most of the other arguments are not used

[oxinabox]

Once CI is unbroken (#127 ) I will hopefully merge this.
But it doesn't seem pressing enough to merge without CI

[codecov]

Codecov Report

Patch coverage: 62.96% and project coverage change: -1.51 ⚠️

Comparison is base (5b3a846) 50.50% compared to head (0aee7b4) 49.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #121      +/-   ##
==========================================
- Coverage   50.50%   49.00%   -1.51%     
==========================================
  Files          27       27              
  Lines        2651     2661      +10     
==========================================
- Hits         1339     1304      -35     
- Misses       1312     1357      +45     

Impacted Files	Coverage Δ
src/Diffractor.jl	100.00% <ø> (ø)
src/stage1/generated.jl	74.19% <ø> (+1.38%)	⬆️
src/stage1/recurse.jl	96.41% <ø> (+0.13%)	⬆️
src/stage1/recurse_fwd.jl	84.00% <ø> (-5.48%)	⬇️
src/stage1/termination.jl	45.45% <18.18%> (-13.64%)	⬇️
src/codegen/forward_demand.jl	29.29% <66.66%> (ø)
src/stage1/forward.jl	44.37% <100.00%> (-22.30%)	⬇️

... and 6 files with indirect coverage changes

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