Pre-CR clean-up of ARIA Graphics spec

On last week's ARIA working group telcon, it was decided to ask the group
to advance the Graphics ARIA and Graphics AAM specs to Candidate
Recommendation status:
https://www.w3.org/2018/03/01-aria-minutes.html#item02

There were a few outstanding clean-up edits I had on my ToDo list for the
Graphics ARIA spec, and a few more that I discovered when I started working
on them.  Those were pushed to the repo yesterday, so if you reviewed the
Editor's Draft last week it will have changed slightly:
https://w3c.github.io/graphics-aria/

The edits don't affect any normative aspects of these specs; they mostly
involve cleaning up the examples & ReSpec formatting.  One of the changes
closes the only open issue on either spec, about avoiding aria-flowto (
https://github.com/w3c/graphics-aria/issues/1). I also removed an editor's
note about adding a more detailed canvas example, since the canvas hit
regions spec has been rolled back.

See the commit messages for more details: https://github.com/
w3c/graphics-aria/commits/master

For working group members, the Call for Consensus on the CR transition
request is still active on the admin mailing list.

~Amelia