cc.Vec2 signAngle bug?

cc.Vec2 signAngle bug?
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#1

Seems cc.Vec2 signAngle behaves differently from API document, at Cocos Creator v1.6.1.
http://cocos2d-x.org/docs/api-ref/creator/v1.5/classes/Vec2.html

It should return from -MathPI to Math.PI, but sometimes returning over the limit.
ex:
returns: 5.302326843376848 v_1:(-512.16, 194.18) v_2:(-274.57, -195.38)
returns: 4.057998878913847 v_1:(-493.00, 134.00) v_2:(124.91, -304.74)
returns: -4.169044791828796 v_1:(105.18, -472.16) v_2:(-98.61, 33.13)

It seems working correctly, if add this.

if (returns > Math.PI) {
    jreturns -= Math.PI * 2; 
}
else if (returns < -Math.PI) {
    returns += Math.PI * 2;
}

or need to update document.