add some clarifications to feedback docstring

Author: baggepinnen

lib/ControlSystemsBase/src/connections.jl

@@ -303,9 +303,9 @@ The returned closed-loop system will have a state vector comprised of the state
       w2─────►│              ├───────►z2
               └──────────────┘
 ```
-`U1`, `W1` specifies the indices of the input signals of `sys1` corresponding to `u1` and `w1`
-`Y1`, `Z1` specifies the indices of the output signals of `sys1` corresponding to `y1` and `z1`
-`U2`, `W2`, `Y2`, `Z2` specifies the corresponding signals of `sys2` 
+- `U1`, `W1` specify the indices of the input signals of `sys1` corresponding to `u1` and `w1`. `W1` contains the indices of the inputs of `sys1` that are included among the inputs to the returned system, i.e., external inputs.
+- `Y1`, `Z1` specify the indices of the output signals of `sys1` corresponding to `y1` and `z1`. `Z1 contains the indices of the outputs of `sys1` that are included among the outputs of the returned system, i.e., external outputs.
+- `U2`, `W2`, `Y2`, `Z2` specify the corresponding signals of `sys2`. `W2 contains the indices of the inputs of `sys2` that are included among the inputs to the returned system, i.e., external inputs. `Z2` contains the indices of the outputs of `sys2` that are included among the outputs of the returned system, i.e., external outputs.
 
 Specify  `Wperm` and `Zperm` to reorder the inputs (corresponding to [w1; w2])
 and outputs (corresponding to [z1; z2]) in the resulting statespace model.
@@ -314,7 +314,7 @@ Negative feedback (α = -1) is the default. Specify `pos_feedback=true` for posi
 
 See also `lft`, `starprod`, `sensitivity`, `input_sensitivity`, `output_sensitivity`, `comp_sensitivity`, `input_comp_sensitivity`, `output_comp_sensitivity`, `G_PS`, `G_CS`.
 
-The manual section [From block diagrams to code](https://juliacontrol.github.io/ControlSystems.jl/stable/man/creating_systems/#From-block-diagrams-to-code) contains higher-level instructions on how to use this function.
+The manual section [From block diagrams to code](https://juliacontrol.github.io/ControlSystems.jl/stable/man/creating_systems/#From-block-diagrams-to-code) contains higher-level instructions on how to use this function. See also [RobustAndOptimalControl.jl: Connections using named signals](https://juliacontrol.github.io/RobustAndOptimalControl.jl/dev/#Connecting-systems-together) for a higher-level interface.
 
 See Zhou, Doyle, Glover (1996) for similar (somewhat less symmetric) formulas.
 """