Improve the qcomm aot part docs (#5868)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/5868

the schema file copy is an actual AOT step. It's a workaround but it has been here a while, probably easier to have it as part of the build script instead of manually doing it everytime

Reviewed By: kirklandsign

Differential Revision: D63878765

fbshipit-source-id: 7fc4b848b72e2859364e21cbea3dd1b433d6cd8e