Add and export documentation for ol.render.canvas.Immediate

This PR copies and expands on documentation from `ol.render.IVectorContext` for `ol.render.canvas.Immediate` and adds exports needed for the documentation to appear in the generated documentation.

Currently, the constructor is exported to ensure the documentation gets generated, but ideally it would be hidden since this object is not intended to be created by a user.  I'm not sure if it's possible to generate documentation without exporting the constructor.

Also, there is no place that actually links to this documentation at the moment.  Ideally, documentation for the `postcompose` event will reference it when I finish the documentation of events.