Update docs/src/extensions.md

Co-authored-by: Ronny Bergmann <[email protected]>

docs/src/extensions.md

@@ -57,7 +57,7 @@ Manopt.max_stepsize(::FixedRankMatrices, ::Any)
 ## JuMP.jl
 
 Manopt can be used using the [JuMP.jl](https://github.com/jump-dev/JuMP.jl) interface.
-The manifold is provided in the `@variable` macro (note that only array manifolds are supported).
+The manifold is provided in the `@variable` macro. Note that until now, only variables (points on manifolds) are supported, that are arrays, i.e. especially structs do not yet work.
 The algebraic expression of the objective function is specified in the `@objective` macro.
 The `descent_state_type` attribute specifies the solver.