Top level docs are mis-placed or missing in most files

Per [go/code-snippets-style#descriptive-top-level-comment](http://goto.google.com/code-snippets-style#descriptive-top-level-comment), we should have top level comments that are placed outside of the region tags, to describe the snippets.

From a brief walk through some parts of the repo, I'm noticing most files don't have the top level comment at all, and I also a noticed a number of them, particularly in `storage/transfer_service` where they are wrongly placed inside the bounds of the region tag, causing them to render in the c.g.c docs, which doesn't seem is the desirable functionality.