[Doc] Clarify that variance estimor is biaised for normalization layers (#39752)
Summary:
Closes https://github.com/pytorch/pytorch/issues/39330
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39752
Differential Revision: D21980097
Pulled By: ngimel
fbshipit-source-id: 2bdcb8bf8194768985f5a8787712d215c0c5c1ec
