More comprehensive docs for ol.DeviceOrientation

Adding an extended class description of DeviceOrientation along with
some additional details in the property accessors.
Update based on comments

This removes @class and fixes the return type pattern to match the
project standard.  There are a couple of minor changes to the text as
well.
Use markdown format for inline links
Merge pull request #978 from pagameba/doc-deviceorientation

More comprehensive docs for ol.DeviceOrientation