add more details in documentation of geometry
add more details in documentation of geometry

mit-crpg/openmc,liangjg/openmc,mit-crpg/openmc,shikhar413/openmc,walshjon/openmc,paulromano/openmc,shikhar413/openmc,paulromano/openmc,mit-crpg/openmc,shikhar413/openmc,liangjg/openmc,shikhar413/openmc,paulromano/openmc,walshjon/openmc,smharper/openmc,liangjg/openmc,paulromano/openmc,walshjon/openmc,amandalund/openmc,smharper/openmc,walshjon/openmc,amandalund/openmc,liangjg/openmc,smharper/openmc,mit-crpg/openmc,smharper/openmc,amandalund/openmc,amandalund/openmc