Improve docs.

Author: dourouc05

src/Chuffed.jl

@@ -13,7 +13,7 @@ module FZN
 include("FlatZincOptimizer.jl")
 end
 
-function Optimizer(; stdin::IO=stdin, stdout::IO=stdout,)
+function Optimizer(; stdin::IO=stdin, stdout::IO=stdout)
     return FZN.Optimizer(Chuffed_jll.run_chuffed; stdin=stdin, stdout=stdout)
 end
 

src/FlatZincOptimizer.jl

@@ -101,7 +101,8 @@ end
 """
     _solver_command(x::Union{Function, String})
 
-Functionify the solver command so it can be called as follows:
+Functionify the solver command as an [`AbstractFznSolverCommand`](@ref) object,
+so it can be called as follows:
 
 ```julia
 foo = _solver_command(x)