Added description of linear congruential PRNG in documentation.
Added description of linear congruential PRNG in documentation.

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