What is the proper way to build and support multi-architecture snap in store (year 2018)?

No

The link I read and provided (https://docs.snapcraft.io/build-snaps/syntax#architectures) clearly says:

The snapcraft.yaml file is the main entry point to create a snap through Snapcraft.

This has been my ‘goto’ for all snapcraft related RTM issues.

The confusion is that there is a forum post with the tag doc here:

This seems to be more up to date than the manual?