Clarify Platform Capability Descriptor's optional status

[nunojpg]

I think the specification leaves unclear that the WebUSB Platform Capability Descriptor is not mandatory for the browser to access the device.

Afaik the Platform Capability Descriptor is as of now only used for Landing Page notifications, and eventually might be used for Operating Systems to provide adequate permissions for the browser to access the device (Linux where browser is not root for eg).

Notes on other prospective uses might be useful for device manufacturers to decide if to include or not the WebUSB Platform Capability Descriptor.

[reillyeon]

Thank you for filing this issue. I will attempt to make the specification clearer on these points.