Added derivation for reflection off of cones in documentation.
Added derivation for reflection off of cones in documentation.

wbinventor/openmc,mjlong/openmc,shenqicang/openmc,sxds/opemmc,wbinventor/openmc,samuelshaner/openmc,amandalund/openmc,mjlong/openmc,smharper/openmc,shenqicang/openmc,lilulu/openmc,shikhar413/openmc,wbinventor/openmc,walshjon/openmc,paulromano/openmc,samuelshaner/openmc,liangjg/openmc,paulromano/openmc,keadyk/openmc_mg_prepush,bhermanmit/openmc,keadyk/openmc_mg_prepush,paulromano/openmc,shikhar413/openmc,mit-crpg/openmc,samuelshaner/openmc,lilulu/openmc,liangjg/openmc,shikhar413/openmc,mit-crpg/openmc,amandalund/openmc,smharper/openmc,bhermanmit/openmc,kellyrowland/openmc,walshjon/openmc,liangjg/openmc,smharper/openmc,wbinventor/openmc,nhorelik/openmc,johnnyliu27/openmc,nhorelik/openmc,paulromano/openmc,walshjon/openmc,johnnyliu27/openmc,lilulu/openmc,mit-crpg/openmc,kellyrowland/openmc,keadyk/openmc_mg_prepush,mit-crpg/openmc,johnnyliu27/openmc,shikhar413/openmc,liangjg/openmc,amandalund/openmc,sxds/opemmc,samuelshaner/openmc,amandalund/openmc,smharper/openmc,johnnyliu27/openmc,walshjon/openmc,bhermanmit/cdash