Proposing marking obsolescence on types in documentation

[benguild]

I’ve been playing around with S2 and wanted to suggest updating the documentation (perhaps via a pull request) to highlight that S2Polygon is considered obsolete on the page that describes it: http://s2geometry.io/devguide/basic_types

That only seems to be mentioned elsewhere such as here: http://s2geometry.io/devguide/coding_style

It may be helpful for folks to see that when looking at information about those types.