Document that options in layers and properties in features are set as object properties

Fixes #1199. Couldn't think of any way of 'inheriting' this doc, so repeated on all layers (except layer.Layer). Also document that feature properties are treated likewise
Suggest adding an example as above, perhaps:

> For example, if you include `foo: 'bar'` in the options then you can retrieve this value later using `layer.get('foo');`