Update pool.md
Update pool.md

Changed wording to emphasize that without a mixin, even if it's the default value (though the outcome of this is undefined, so not going to mention anything there).
RSpace/aframe,weddingdj/aframe,ngokevin/aframe,jaxmolina/ar,RSpace/aframe,weddingdj/aframe,wizgrav/aframe,jaxmolina/ar,chenzlabs/aframe,darkwing/aframe,darkwing/aframe,dmarcos/aframe,chenzlabs/aframe,mkungla/aframe,mkungla/aframe,dmarcos/aframe,googlecreativelab/aframe,ngokevin/aframe,aframevr/aframe,googlecreativelab/aframe,aframevr/aframe,wizgrav/aframe