update callback docstring

Author: MaxenceGollier

src/LMTR_alg.jl

@@ -147,21 +147,7 @@ The algorithm stops either when `√(ξₖ/νₖ) < atol + rtol*√(ξ₀/ν₀)
 The value returned is a `GenericExecutionStats`, see `SolverCore.jl`.
 
 # Callback
-The callback is called at each iteration.
-The expected signature of the callback is `callback(nlp, solver, stats)`, and its output is ignored.
-Changing any of the input arguments will affect the subsequent iterations.
-In particular, setting `stats.status = :user` will stop the algorithm.
-All relevant information should be available in `nlp` and `solver`.
-Notably, you can access, and modify, the following:
-- `solver.xk`: current iterate;
-- `solver.∇fk`: current gradient;
-- `stats`: structure holding the output of the algorithm (`GenericExecutionStats`), which contains, among other things:
-  - `stats.iter`: current iteration counter;
-  - `stats.objective`: current objective function value;
-  - `stats.solver_specific[:smooth_obj]`: current value of the smooth part of the objective function;
-  - `stats.solver_specific[:nonsmooth_obj]`: current value of the nonsmooth part of the objective function;
-  - `stats.status`: current status of the algorithm. Should be `:unknown` unless the algorithm has attained a stopping criterion. Changing this to anything other than `:unknown` will stop the algorithm, but you should use `:user` to properly indicate the intention;
-  - `stats.elapsed_time`: elapsed time in seconds.
+$(callback_docstring)
 """
 function LMTR(
   nls::AbstractNLSModel,