Add docs for outer boxconstrained options

docs/src/user/config.md

@@ -54,6 +54,16 @@ In addition to the solver, you can alter the behavior of the Optim package by us
 * `callback`: A function to be called during tracing. A return value of `true` stops the `optimize` call. The callback function is called every `show_every`th iteration. If `store_trace` is false, the argument to the callback is of the type  [`OptimizationState`](https://github.com/JuliaNLSolvers/Optim.jl/blob/a1035134ca1f3ebe855f1cde034e32683178225a/src/types.jl#L155), describing the state of the current iteration. If `store_trace` is true, the argument is a list of all the states from the first iteration to the current. 
 * `time_limit`: A soft upper limit on the total run time. Defaults to `NaN` (unlimited).
 
+Box constrained optimization has additional keywords to alter the behavior of the outer solver:
+
+* `outer_x_tol`: Absolute tolerance in changes of the input vector `x`, in infinity norm. Defaults to `0.0`.
+* `outer_f_tol`: Relative tolerance in changes of the objective value. Defaults to `0.0`.
+* `outer_g_tol`: Absolute tolerance in the gradient, in infinity norm. Defaults to `1e-8`. For gradient free methods, this will control the main convergence tolerance, which is solver specific.
+* `allow_outer_f_increases`: Allow steps that increase the objective value. Defaults to `false`. Note that, when setting this to `true`, the last iterate will be returned as the minimizer even if the objective increased.
+* `outer_iterations`: How many iterations will run before the algorithm gives up? Defaults to `1_000`.
+
+If you specify `outer_iterations = 10` and `iterations = 100`, the outer algorithm will run for `10` iterations, and for each outer iteration the inner algorithm will run for `100` iterations.
+
 We currently recommend the statically dispatched interface by using the `Optim.Options`
 constructor:
 ```jl