Update devstack documentation
Update devstack documentation

It no longer makes much sense to run the XBlock synchronously in a
devstack, so remove documentation to that effect.

arbrandes/hastexo-xblock,hastexo/hastexo-xblock,hastexo/hastexo-xblock,arbrandes/hastexo-xblock,arbrandes/hastexo-xblock,arbrandes/hastexo-xblock,arbrandes/hastexo-xblock,hastexo/hastexo-xblock,hastexo/hastexo-xblock