Add 'timer' api docs (#339)

libs/core/timer.ts

@@ -11,36 +11,40 @@ namespace control {
         }
 
         /**
-         * Gets the elapsed time in millis since the last reset
+         * Get the elapsed time in millis since the last reset
          */
         //% blockId=timerMillis block="%timer|millis"
+        //% help=control/timer/millis
         millis(): number {
             return control.millis() - this.start;
         }
         
         /**
-         * Gets the elapsed time in seconds since the last reset
+         * Get the elapsed time in seconds since the last reset
          */
         //% blockId=timerSeconds block="%timer|seconds"
+        //% help=control/timer/seconds
         seconds(): number {
             return this.millis() / 1000;
         }
 
         /**
-         * Resets the timer
+         * Reset the timer
          */
         //% blockId=timerRest block="%timer|reset"
+        //% help=control/timer/reset
         reset() {
             this.start = control.millis();
         }
 
         /**
-         * Pauses until the timer reaches the given amount of milliseconds
+         * Pause until the timer reaches the given amount of milliseconds
          * @param ms how long to pause for, eg: 5, 100, 200, 500, 1000, 2000
          */
         //% blockId=timerPauseUntil block="%timer|pause until (ms) %ms"
+        //% help=control/timer/pause-until
         pauseUntil(ms: number) {
-            const remaining = this.millis() - ms;
+            const remaining = ms - this.millis();
             pause(Math.max(0, remaining));
         }
     }