Update depletion documentation to mention fixed-source mode
Update depletion documentation to mention fixed-source mode

shikhar413/openmc,paulromano/openmc,mit-crpg/openmc,mit-crpg/openmc,liangjg/openmc,paulromano/openmc,mit-crpg/openmc,walshjon/openmc,paulromano/openmc,paulromano/openmc,shikhar413/openmc,shikhar413/openmc,walshjon/openmc,amandalund/openmc,amandalund/openmc,liangjg/openmc,walshjon/openmc,liangjg/openmc,walshjon/openmc,amandalund/openmc,liangjg/openmc,mit-crpg/openmc,amandalund/openmc,shikhar413/openmc