Add note to serialization docs about zipfile format (#40288)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/40288
Test Plan: Imported from OSS
Differential Revision: D22140324
Pulled By: jamesr66a
fbshipit-source-id: 01d7aa642ed2f4e4bdac4b7f3223bf4d7e62fd4d
