Fixed messup

I realized I changed the docs for impulseplot, thinking that it was the docs for stepplot, in my last commit. This is now fixed (in accordance with docs for step, not yet in accordance with impulse. Waiting to hear @olof3's thoughs on this phrasing before changing docs for impulse)

src/plotting.jl

@@ -162,19 +162,15 @@ end
 @userplot Impulseplot
 """
     stepplot(sys[, tfinal[,  dt]])
-Plot step response of `sys` with optional final time `tfinal` and discretization time `dt`.
-If not defined, suitable values are chosen based on `sys`.
-The step on the input goes from 0 to 1 and occurs at t=0.
+Plot the step response of system `sys` to a unit step at time `t = 0`.
+If the final time `tfinal` and discretization time `dt` are not prodivded, suitable values are chosen based on `sys`.
 """
 stepplot
 
 """
     impulseplot(sys[, tfinal[,  dt]])
-Plot the impulse response of system `sys` to a unit step at time `t = 0`.
+Plot the impulse response of system `sys`. The impulse is such that the area under it is 1, with the shortest possible timestep.
 If the final time `tfinal` and discretization time `dt` are not prodivded, suitable values are chosen based on `sys`.
-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.
 """
 impulseplot