improve motor docs

Add a paragraph to explain when attributes are handled.

See ev3dev/ev3dev#558

Author: ensonic

spec.json

@@ -11,6 +11,11 @@
                 "positional and directional feedback such as the EV3 and NXT motors.",
                 "This feedback allows for precise control of the motors. This is the",
                 "most common type of motor, so we just call it `motor`."
+                ""
+                "The way to configure a motor is to set the '_sp' attributes when",
+                "calling a command or before. Only in 'run_direct' mode attribute",
+                "changes are processed immediately, in the other modes they only",
+                "take place when a new command is issued."
                 ],
             "docsLink": "http://www.ev3dev.org/docs/drivers/tacho-motor-class/",
             "systemClassName":  "tacho-motor",