Update optimization.md (#350)

docs/optimization.md

@@ -22,8 +22,8 @@ print(recommendation)
 `recommendation` holds the optimal attributes `args` and `kwargs` found by the optimizer for the provided function.
 In this example, the optimal value will be found in `recommendation.args[0]` and will be a `np.ndarray` of size 2.
 
-`instrumentation=n` is a shortcut to state that the function has only one variable, of dimension `n`,
-Defining the following instrumentation instead will optimize on both `x` and `y`.
+`instrumentation=n` is a shortcut to state that the function has only one variable, continuous, of dimension `n`,
+Defining the following instrumentation instead will optimize on both `x` (continuous, dimension 2) and `y` (continuous, dimension 1).
 ```python
 instrum = ng.Instrumentation(ng.var.Array(2), y=ng.var.Array(1).asscalar())
 optimizer = ng.optimizers.OnePlusOne(instrumentation=instrum, budget=100)