Document use of conditionals for sparsity detection

With the new `TracerSparsityDetector` from SparseConnectivityTracer, `if` blocks are not supported if they involve an input variable. They must be replaced with `ifelse` statements, which are compatible with operator overloading.

In your objective functions or constraints, do not write

```julia
y = if x[1] > 0
x[2]
else
x[3]
end
```

but instead write

```julia
y = ifelse(x[1] > 0, x[2], x[3])
```

Related:

- https://github.com/JuliaSmoothOptimizers/ADNLPModels.jl/issues/247#issuecomment-2207029473