Document timeout unit, add non_blocking read/write interrupts

[flukejones]

I'd been using micros for the timeout of an interrupt read and this was causing some very very janky behaviour that I was struggling to debug. The docs don't mention the unit required for timeouts and it seems 1 millisecond is the lowest I can go.

It would also be nice to have a non-blocking read/write_interrupt call.

[a1ien]

Non-blocking mode in it's hard implement using async, await and future. You can read this #3 (comment).

I add info about timeout in documentation.

Also you can try use unsafe libusb1-sys crate and get async function from native libusb
5a38c32
This old async code

[zoumi]

If the USB device send data to PC while a read timeout happening, the data will be discarded?

[a1ien]

If the USB device send data to PC while a read timeout happening, the data will be discarded?

This can't happen. Becouse all transaction start by host. If host not request device can't anything send