Add docstrings to state and stats

src/stats_state.jl

@@ -1,4 +1,20 @@
+"""
+$(TYPEDEF)
 
+Stores the optimization run's statistics that is returned 
+in the `stats` field of the `OptimizationResult`. 
+
+## Fields
+- `iterations`: number of iterations
+- `time`: time taken to run the solver
+- `fevals`: number of function evaluations
+- `gevals`: number of gradient evaluations
+- `hevals`: number of hessian evaluations
+
+Default values for all the field are set to 0 and hence even when 
+you might expect non-zero values due to unavilability of the information 
+from the solver it would be 0.
+"""
 struct OptimizationStats
     iterations::Int
     time::Float64
@@ -7,9 +23,24 @@
     hevals::Int
 end
 
-OptimizationStats(; iterations = 0, time = 0.0, fevals = 0, gevals = 0, hevals = 0) =
+function OptimizationStats(; iterations = 0, time = 0.0, fevals = 0, gevals = 0, hevals = 0)
     OptimizationStats(iterations, time, fevals, gevals, hevals)
+end
 
+"""
+$(TYPEDEF)
+
+Stores the optimization run's state at the current iteration 
+and is passed to the callback function as the first argument.
+
+## Fields
+- `iteration`: current iteration
+- `u`: current solution
+- `objective`: current objective value
+- `gradient`: current gradient
+- `hessian`: current hessian
+- `solver_state`: if the solver has its own state object then it is stored here
+"""
 struct OptimizationState{X, O, G, H, S}
     iteration::Int
     u::X
@@ -19,6 +50,7 @@
     solver_state::S
 end
 
-OptimizationState(; iteration = 0, u = nothing, objective = nothing,
-        gradient = nothing, hessian = nothing, solver_state = nothing) =
+function OptimizationState(; iteration = 0, u = nothing, objective = nothing,
+        gradient = nothing, hessian = nothing, solver_state = nothing)
     OptimizationState(iteration, u, objective, gradient, hessian, solver_state)
+end