Cleanup TorchScript rst docs (#18234)
Summary:
* Adds more headers for easier scanning
* Adds some line breaks so things are displayed correctly
* Minor copy/spelling stuff
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18234
Reviewed By: ezyang
Differential Revision: D14567737
Pulled By: driazati
fbshipit-source-id: 046d991f7aab8e00e9887edb745968cb79a29441
