Autogenerate documentation in framework crates #309

Opening this to track implementation work after #264.

Possibilities for having documentation in framework crates include:

1. Links to https://developer.apple.com/
   • At least for classes, done in a8e99f6.
2. Grab documentation from the above and inline it
   • I did some preliminary work in bindgen support #85 (comment) (the files are available locally as well).
3. Convert code comments in the headers to documentation comments
   • Done in 581f098.
4. Use Xamarin's / .NET's
5. ?

Or some combination of these.

Also: Add documentation on which properties are weak and copy (that's really nice to know when developing against such an API).