doc : clearer description of NonLinMPC extended vectors

Author: franckgaga

src/controller/nonlinmpc.jl

@@ -273,10 +273,16 @@ NonLinMPC controller with a sample time Ts = 10.0 s:
     `model` is a [`LinModel`](@ref)).
 
     The economic cost ``J_E`` and custom constraint ``\mathbf{g_c}`` functions receive the
-    extended vectors ``\mathbf{U_e}`` (`nu*Hp+nu` elements), ``\mathbf{Ŷ_e}`` (`ny+ny*Hp`
-    elements) and  ``\mathbf{D̂_e}`` (`nd+nd*Hp` elements) as arguments. They all include the
-    values from ``k`` to ``k + H_p`` (inclusively). They also receives the slack ``ϵ``
-    (scalar), which is always zero if `Cwt=Inf`.
+    extended vectors ``\mathbf{U_e}``, ``\mathbf{Ŷ_e}`` and  ``\mathbf{D̂_e}`` as arguments. 
+    They also receives the slack ``ϵ`` (scalar), which is always zero if `Cwt=Inf`. The 
+    following table details the vector sizes and the time steps of the first and last data
+    point in them:
+
+    | VECTOR           | SIZE           | FIRST TIME STEP | LAST TIME STEP |
+    | :--------------- | :------------- | :-------------- | :------------- |
+    | ``\mathbf{U_e}`` | `(nu*(Hp+1),)` | ``k + 0``       | ``k + H_p``    |
+    | ``\mathbf{Ŷ_e}`` | `(ny*(Hp+1),)` | ``k + 0``       | ``k + H_p``    |
+    | ``\mathbf{D̂_e}`` | `(nd*(Hp+1),)` | ``k + 0``       | ``k + H_p``    |
     
     More precisely, the last two time steps in ``\mathbf{U_e}`` are forced to be equal, i.e.
     ``\mathbf{u}(k+H_p) = \mathbf{u}(k+H_p-1)``, since ``H_c ≤ H_p`` implies that