[doc] Add overflow notice for cuFFT on half precision (#35594)
Summary:
This would fix https://github.com/pytorch/pytorch/issues/33485.
cc ptrblck
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35594
Differential Revision: D20722426
Pulled By: ngimel
fbshipit-source-id: 68f7304de5d6cecdd9e34e8697fc84bc551b1a45
