Correct docs for Phaser.Math.GetSpeed()

Fixes phaserjs#4895

Author: samme

src/math/GetSpeed.js

@@ -5,15 +5,19 @@
  */
 
 /**
- * Calculate the speed required to cover a distance in the time given.
+ * Calculate a per-ms speed from a distance and time (given in seconds).
  *
  * @function Phaser.Math.GetSpeed
  * @since 3.0.0
  *
- * @param {number} distance - The distance to travel in pixels.
- * @param {integer} time - The time, in ms, to cover the distance in.
+ * @param {number} distance - The distance.
+ * @param {integer} time - The time, in seconds.
  *
- * @return {number} The amount you will need to increment the position by each step in order to cover the distance in the time given.
+ * @return {number} The speed, in distance per ms.
+ *
+ * @example
+ * // 400px over 1 second is 0.4 px/ms
+ * Phaser.Math.GetSpeed(400, 1) // -> 0.4
  */
 var GetSpeed = function (distance, time)
 {