updated documentation geometry.md
updated documentation geometry.md

added missing documentation on segments for plane and box primitives in geometry.md

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