doc: details on initstate! for StateEstimator

Author: franckgaga

src/estimator/execute.jl

@@ -87,7 +87,8 @@ end
 Init `estim.x̂0` states from current inputs `u`, measured outputs `ym` and disturbances `d`.
 
 The method tries to find a good steady-state for the initial estimate ``\mathbf{x̂}``. It
-removes the operating points with [`remove_op!`](@ref) and call [`init_estimate!`](@ref):
+stores `u - estim.model.uop` at `estim.lastu0` and removes the operating points and with 
+[`remove_op!`](@ref), and call [`init_estimate!`](@ref):
 
 - If `estim.model` is a [`LinModel`](@ref), it finds the steady-state of the augmented model
   using `u` and `d` arguments, and uses the `ym` argument to enforce that