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
Add "WebGL only" note to bind-input example
Merge pull request #985 from ebelo/patch-1

add webgl only info
Bind to layergroup instead of layers
Merge pull request #981 from fredj/side-by-side

Side-by-side example broken
Export ol.*.SUPPORTED symbols
Better user notification if WebGL is not supported
Merge pull request #990 from fredj/examples_no_webgl

Better user notification if WebGL is not supported
Add ol.DeviceMotion