Added documentation on confidence intervals.
Added documentation on confidence intervals.

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