Add constructors documentation in docstrings

I noted most of the structures involved in #72 have a docstring, which is great! For instance https://github.com/JuliaSmoothOptimizers/PartiallySeparableNLPModels.jl/blob/3266fd6f0ed18a3ae1aad6e4ad11249ee99b9296/src/partitionedNLPModels/PBFGSNLPModels.jl#L11

I think it would be even better if the constructors available for the structure would be documented as well.