document merging mixin properties (fixes #1261)
document merging mixin properties (fixes #1261)

MatthewShotton/aframe,fernandojsg/aframe,dmarcos/aframe,jzitelli/aframe,jzitelli/aframe,MatthewShotton/aframe,RSpace/aframe,weddingdj/aframe,RSpace/aframe,weddingdj/aframe,ngokevin/aframe,googlecreativelab/aframe,msimpson/aframe,msimpson/aframe,mkungla/aframe,jaxmolina/ar,wizgrav/aframe,chenzlabs/aframe,aframevr/aframe,mkungla/aframe,wizgrav/aframe,fernandojsg/aframe,darkwing/aframe,chenzlabs/aframe,ngokevin/aframe,darkwing/aframe,aframevr/aframe,dmarcos/aframe,jaxmolina/ar,googlecreativelab/aframe