[FX] Add note about more use cases of FX (#51576)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/51576
Test Plan: Imported from OSS
Reviewed By: ansley
Differential Revision: D26203610
Pulled By: jamesr66a
fbshipit-source-id: d33a3e7e0f3a959349ed0e29a1aba0592022606d
