Document the distribution of `Phaser.Math.RND.weightedPick` inside its JSDoc

[Bertie690]

Currently, the weightedPick function fails to mention precisely how much earlier items in the array are favored over later ones, which is a fairly important detail to simply omit without warning.

Admittedly, the provided code isn't the clearest in the world either...

    weightedPick: function (array)
    {
        return array[~~(Math.pow(this.frac(), 2) * (array.length - 0.5) + 0.5)];
    },