update symeig backward note about similar eigenvalues (#52311)
Summary:
First part of https://github.com/pytorch/pytorch/issues/49886 to at least properly warn users of the current state
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52311
Reviewed By: soulitzer
Differential Revision: D26495644
Pulled By: albanD
fbshipit-source-id: 72abdfe41cdbcc1ac739a536eb85d1aa4ba90897
