[FX][docs] Document gotcha about training flag (#68915)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/68913
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68915
Reviewed By: jamesr66a
Differential Revision: D32705410
Pulled By: jubinchheda
fbshipit-source-id: a44c17ab0e62465823ceb0ef983ae330b50fb073
