Add description of <threads> element in user's guide.
Add description of <threads> element in user's guide.

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