mathrole/notation/semantic -> intent

As per my action item from today's meeting, I updated the .md files in the
mathml/docs directory to use 'intent' and '@' in the examples. I caught
several other textual places where the changes made sense, but I'm sure
there are many others.

Some of Sam's docs used a combination of 'notation' and 'meaning' in them
-- I did *not* change them. I strongly recommend those who authored those
docs read through them again and update anything you think is appropriate.
They are on the table for discussion next week.