[docs] add example of how to register function with multiple input arguments

[odow]

Closes #3253

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: -0.92 ⚠️

Comparison is base (7bd8e09) 98.10% compared to head (27235b6) 97.19%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3258      +/-   ##
==========================================
- Coverage   98.10%   97.19%   -0.92%     
==========================================
  Files          34       34              
  Lines        4702     4663      -39     
==========================================
- Hits         4613     4532      -81     
- Misses         89      131      +42     

Impacted Files	Coverage Δ
src/precompile.jl	0.00% <0.00%> (-100.00%)	⬇️
src/Containers/vectorized_product_iterator.jl	90.90% <0.00%> (-4.55%)	⬇️
src/Containers/DenseAxisArray.jl	93.10% <0.00%> (-2.76%)	⬇️
src/JuMP.jl	95.32% <0.00%> (-1.50%)	⬇️
src/macros.jl	97.27% <0.00%> (-1.31%)	⬇️
src/solution_summary.jl	96.42% <0.00%> (-1.20%)	⬇️
src/aff_expr.jl	96.12% <0.00%> (-1.17%)	⬇️
src/nlp.jl	98.96% <0.00%> (-1.04%)	⬇️
src/Containers/Containers.jl	90.90% <0.00%> (-0.76%)	⬇️
src/sd.jl	99.30% <0.00%> (-0.70%)	⬇️
... and 5 more

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[odow]

Preview: https://jump.dev/JuMP.jl/previews/PR3258/manual/nlp/#Registered-functions-with-a-variable-number-of-arguments