Clarify the wording around `USBEndpoint.packetSize`

[ArcaneNibble]

The WebUSB specification says:

The packetSize attribute declares the packet size employed by this endpoint and MUST be equal to the value of the wMaxPacketSize of the endpoint descriptor defining it. In a High-Speed, High-Bandwidth endpoint this value will include the multiplication factor provided by issuing multiple transactions per microframe. In a SuperSpeed device this value will include the multiplication factor provided by the bMaxBurst field of the SuperSpeed Endpoint Companion descriptor.

This field cannot simultaneously be equal to the value of the descriptor and also include the relevant multiplication factors.

[reillyeon]

I believe the implementation in Chromium is currently implementing the first half of the requirement without considering the multipliers. It hasn't come up in practice to my knowledge because High-Speed, High-Bandwidth endpoints are relatively rare.

The key thing for developers using the API is that the packet size informs how data should be chunked since devices may consider a transfer complete when a packet with less than the maximum packet size is transmitted. I need to do additional research to recall how that applies to these packet bursts supported by High-Speed and SuperSpeed devices.