Improve documentation for step and impulse

src/timeresp.jl

@@ -7,6 +7,7 @@
 Calculate the step response of system `sys`. If the final time `Tf` or time
 vector `t` is not provided, one is calculated based on the system pole
 locations.
+The step on the input goes from 0 to 1 and occurs at t=0.
 
 `y` has size `(length(t), ny, nu)`, `x` has size `(length(t), nx, nu)`"""
 function Base.step(sys::StateSpace, t::AbstractVector; method=:cont)
@@ -36,6 +37,9 @@ Base.step(sys::TransferFunction, t::AbstractVector; kwargs...) = step(ss(sys), t
 Calculate the impulse response of system `sys`. If the final time `Tf` or time
 vector `t` is not provided, one is calculated based on the system pole
 locations.
+The the impulse is such that the area under it is 1, with the shortest possible time-step.
+For continuos systems this is the Dirac delta function.
+For discrete systems this is the Kronecker delta function.
 
 `y` has size `(length(t), ny, nu)`, `x` has size `(length(t), nx, nu)`"""
 function impulse(sys::StateSpace, t::AbstractVector; method=:cont)