Add note in Chain.from_endf docstring about missing FPY
Add note in Chain.from_endf docstring about missing FPY

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