[DOC beta] Fix warnings generated by yuidoc.

Currently yuidoc generates a handfull of warnings when run on the Ember code base. See https://github.com/emberjs/website/blob/master/data/api.yml#L27189-L27236. 

This pr fixes all but 2 of these warnings. The remaining warnings complain about the @version tag being non standard and one warning from RSVP which will need to be fixed upstream.