move migration guide to appendix (#31068)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31068
Let's get it out of the early parts now that the recursive API has been
around for a while
Test Plan: Imported from OSS
Differential Revision: D18920498
Pulled By: suo
fbshipit-source-id: 6f4389739dd9e7e5f3014811b452249cc21d88e7
