Autogenerate documentation in framework crates

[madsmtm]

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).

[madsmtm]

Adding documentation for weak properties was done in 27ab005, and linking to Apple's documentation was done in a8e99f6, and auto-generating from documentation was done in 581f098.

[madsmtm]

The auto-generated documentation is still pretty bad though, we need to either:

• Build a crate capable of properly parsing the semi-Sphinx syntax that Apple's headers use.
• Work to improve this parser in upstream libclang.

With the latter being my preference, but probably also being a lot harder.