Add documentation on `solver_specific` in benchmark

[tmigot]

I think there are two possibilities for this.

i) Update the file https://github.com/JuliaSmoothOptimizers/JSOTutorials.jl/blob/main/tutorials/introduction-to-solverbenchmark/index.jmd to add some comments on how the package handles solver_specific in the stats.

See #149 for an illustration.

There is no actual test of bmark_solvers in the tutorial, so suggestions are welcome to bring this topic. For instance, it is possible to add a simple benchmark at the end (or in a new tutorial that shows more of the SolverBenchmark functions ?) using NLPModelsTest and JSOSolvers.

ii) Add this information in the docstrings in the package.