Documentation: Make it easier to find styling options for features

[wphicks]

It would be helpful to make it easier to find available styling options for features. As a specific example, if I look at the documentation for polygonFeature, I don't see what style options are available for polygons.

[manthey]

Some of this was improved in PR #819, so at least the styles are enumerated in the geo.polygonFeature.spec, though this hasn't been published to github.io yet.

For polygons (and features that subclass the style function), we could put a link to the appropriate typedef (or, less optimally, duplicate the docs from that typedef). For features where the style function is inherited, this wouldn't be directly possible.

@wphicks, do you think a link to the appropriate typedef is sufficient, or do you think that duplicating the documentation would be preferred?

[wphicks]

I think a link to the appropriate typedef would be perfect. So long as someone who starts by searching for "style" knows where to go, that should work fine.