Update ResultsList.get_eigenvalues docstring
Update ResultsList.get_eigenvalues docstring

Add mention of the returned uncertainties

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