Fix type issues in CaptureDescriptor and CaptureManager, add documentation

[finnvoor]

• Fixes setting/getting MTLCaptureDescriptor outputURL to an NSString instead of an NSURL (https://developer.apple.com/documentation/metal/mtlcapturedescriptor/3237250-outputurl?language=objc)
• Fixes casting () to Result<(), String> in CaptureManager.start_capture
• Adds documentation for capturing a GPU trace to a file without running from Xcode

These issues were causing a panic both when setting a CaptureDescriptor outputURL and when starting a capture using a CaptureDescriptor.

---

Example usage in a wgpu/winit program

// create capture scope using wgpu device
let capture_scope = unsafe {
    device.as_hal::<api::Metal, _, _>(|device| {
        let raw_device = device.unwrap().raw_device().lock();
        metal::CaptureManager::shared().new_capture_scope_with_device(&raw_device)
    })
};
let capture_scope = metal::CaptureManager::shared()
    .new_capture_scope_with_device(&metal::Device::system_default().unwrap());

// inside event loop
Event::RedrawRequested(_) => {
    capture_scope.begin_scope();
    // render work
    capture_scope.end_scope();
}

// when you want to capture a frame to a trace file
let capture_descriptor = metal::CaptureDescriptor::new();
capture_descriptor.set_capture_scope(&capture_scope);
capture_descriptor.set_output_url(std::path::Path::new(
    "~/.../.../framecapture.gputrace",
));
capture_descriptor.set_destination(metal::MTLCaptureDestination::GpuTraceDocument);
metal::CaptureManager::shared().start_capture(&capture_descriptor);
window.request_redraw();