Update docs (#2919)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2919

The note directive on sphinx doesn't render well on markdown. Remove it to avoid cause confusion.

Reviewed By: mergennachin, cccclai

Differential Revision: D55881939

fbshipit-source-id: a4252f0b70593ecd97e5cc352c601e772a9c222a