Use reStructuredText to enable inline docs.

Converts documentation to use reStructuredText.
Currently this shows the description of the function in the autocomplete window.

This doesn't enable param and return information in the Quick Help, but there
doesn't seem to be a way to do that yet.

> @importantshock Protip: use /// for method/class/function-level comments in Swift. Xcode will display the comments inline in autocomplete
> @importantshock Note that unlike ObjC (which uses HeaderDoc), Swift uses reStructured Text.
> https://twitter.com/importantshock/status/477549574978949120