Update ArrowHelper.html

docs/api/extras/helpers/ArrowHelper.html

@@ -32,7 +32,7 @@
 
 		<h3>[name]([page:Vector3 dir], [page:Vector3 origin], [page:Number length], [page:Number hex], [page:Number headLength], [page:Number headWidth] )</h3>
 		<div>
-		dir -- Vector3 -- direction from origin <br />
+		dir -- Vector3 -- direction from origin. Must be a unit vector. <br />
 		origin -- Vector3 <br />
 		length -- scalar <br />
 		hex -- hexadecimal value to define color ex:0xffff00<br />