doc: minor comment on operating point

Author: franckgaga

src/controller/transcription.jl

@@ -42,9 +42,10 @@ operating point ``\mathbf{x̂_{op}}`` (see [`augment_model`](@ref)):
     \mathbf{x̂}_i(k+H_p)   - \mathbf{x̂_{op}}     \end{bmatrix}
 ```
 where ``\mathbf{x̂}_i(k+j)`` is the state prediction for time ``k+j``, estimated by the
-observer at time ``i=k`` or ``i=k-1`` depending on its `direct` flag. This transcription
-method is generally more efficient for large control horizon ``H_c``, unstable or highly
-nonlinear plant models/constraints. 
+observer at time ``i=k`` or ``i=k-1`` depending on its `direct` flag. Note that 
+``\mathbf{X̂_0 = X̂}`` if the operating points is zero, which is typically the case in 
+practice for [`NonLinModel`](@ref). This transcription method is generally more efficient
+for large control horizon ``H_c``, unstable or highly nonlinear plant models/constraints. 
 
 Sparse optimizers like `OSQP` or `Ipopt` are recommended for this method.
 """