Updating documentation on contains.
Updating documentation on contains.

johnnyliu27/openmc,paulromano/openmc,amandalund/openmc,johnnyliu27/openmc,wbinventor/openmc,shikhar413/openmc,mit-crpg/openmc,mit-crpg/openmc,liangjg/openmc,paulromano/openmc,shikhar413/openmc,walshjon/openmc,johnnyliu27/openmc,smharper/openmc,liangjg/openmc,paulromano/openmc,amandalund/openmc,smharper/openmc,smharper/openmc,mit-crpg/openmc,wbinventor/openmc,shikhar413/openmc,paulromano/openmc,amandalund/openmc,shikhar413/openmc,mit-crpg/openmc,johnnyliu27/openmc,liangjg/openmc,walshjon/openmc,walshjon/openmc,smharper/openmc,wbinventor/openmc,wbinventor/openmc,walshjon/openmc,liangjg/openmc,amandalund/openmc