[docs] add deprecation notes to the legacy nonlinear interface docstrings

[odow]

I think this has been in enough use now that we can recommend it more strongly.

Preview: https://jump.dev/JuMP.jl/previews/PR3711/api/JuMP/#@NLconstraint

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.37%. Comparing base (a16b383) to head (e436314).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3711   +/-   ##
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  Coverage   98.37%   98.37%           
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  Files          43       43           
  Lines        5736     5737    +1     
=======================================
+ Hits         5643     5644    +1     
  Misses         93       93           

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[odow]

I wonder if we should also remove the warning:

We've had lots of people use this now, and I've only heard good things.