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
This still sounds a bit awkward, how about something like this:

>  Note that attribute properties are set as properties on the feature object.

We could also add

>  Use `feature.set(propertyName, value)` to create or update a property and `get(propertyName)` to retrieve its value.