update docs

docs/ulab-pid.ipynb

@@ -443,7 +443,7 @@
    "source": [
     "### step\n",
     "\n",
-    "This method takes no arguments, and steps the controller, i.e., takes and converts the data in the inpput buffer, calculates the three terms of the controller's output, and places the data on the output buffer. At the same time, the `value`, `steps`, and `integral` attributes of the controller are updated."
+    "This method takes no arguments, and steps the controller, i.e., takes and converts the data in the inpput buffer, calculates the three terms of the controller's output, and places the data on the output buffer. At the same time, the `value`, `steps`, and `integral` attributes of the controller are updated. Time differences are measured in units of ms; keep this in mind, when adjusting the values of `P`, `I`, and `D`! "
    ]
   },
   {
@@ -486,7 +486,7 @@
    "source": [
     "### float_step\n",
     "\n",
-    "The method takes one, or two arguments, and returns the computed control value. The first argument is the current value of the signal to be controlled, while the time can be supplied in the optional second argument. If that is missing, then a time increment of 1.0 is taken as default. As with [step](#step), all internal variables are updated."
+    "The method takes one, or two arguments, and returns the computed control value. The first argument is the current value of the signal to be controlled, while the time can be supplied in the optional second argument. If that is missing, then a time increment of 1.0 is taken as default. As with [step](#step), all internal variables are updated. Note that since the time difference can be supplied by the user, the units are not fixed, i.e., the values of `P`, `I`, and `D` can arbitrarily chosen."
    ]
   },
   {