Add @todo API and describe ol.geom.Geometry funcs

The `clone()`, `getSimplifiedGeometry()` and `getType()` methods are all
base class methods documented in subclasses.  They should appear in
ol.geom.Geometry in the API docs as well.
Merge pull request #2177 from pagameba/geometry-api

Add @todo api and describe ol.geom.Geometry funcs