Enhance documentation for URL attachments

Enhance documentation for [`Attachment.init(contentsOf:named:sourceLocation:)`](https://developer.apple.com/documentation/testing/attachment/init(contentsof:named:sourcelocation:)). In particular, provide info on what kind of URLs can be passed in, what the initializer awaits on, and how to use this (since there's intentionally no equivalent `record()` overload.)