[docs] add developer docs for unset_ methods

[odow]

Closes #3989

https://jump.dev/JuMP.jl/previews/PR3990/developers/conventions/#unset_-methods

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (73e067d) to head (583f4cc).
Report is 1 commits behind head on master.

Additional details and impacted files

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##            master     #3990   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           43        43           
  Lines         6082      6082           
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  Hits          6082      6082           

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[joaquimg]

We could even add set_silent(::JuMP.Model, ::Bool), and start using it. Not an issue for this PR though.

[odow]

I've avoid changing set_silent because it is very widely used and it felt like unnecessary churn.