[docs] tweak alphaTest docs
[docs] tweak alphaTest docs

RSpace/aframe,ngokevin/aframe,mkungla/aframe,dmarcos/aframe,RSpace/aframe,aframevr/aframe,ngokevin/aframe,chenzlabs/aframe,mkungla/aframe,wizgrav/aframe,chenzlabs/aframe,dmarcos/aframe,aframevr/aframe,wizgrav/aframe