[FX][docs] Add limitations of symbolic tracing (#50638)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/50638
Test Plan: Imported from OSS
Reviewed By: ansley
Differential Revision: D25933780
Pulled By: jamesr66a
fbshipit-source-id: 0aa97ea05203fbcb707b0e947a465e206104b7df
