Update export documentation (#296)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/296

In light of two stage APIs, export documentation neeeds to change.
Update also links to the gh issue, establishing context and the need for the
change, along with highlighting what is the long term plan for export API.

Reviewed By: mergennachin

Differential Revision: D49209531

fbshipit-source-id: 6c9dd56609f663f57090ecbba5bb8e2b5de94411