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usb: Add a note about buffer thread safety.

This is to replace a commit which added locking here but caused some other problems. The idea behind the Buffer class is that a single producer can call pend_write() more than once and it's idempotent, however this is very complex to extend across multiple threads. Signed-off-by: Angus Gratton <angus@redyak.com.au>

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micropython/usb
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`@@ -134,3 +134,15 @@ USB MIDI devices in MicroPython.`
`134`	`134`
`135`	`135`	`The example [midi_example.py](examples/device/midi_example.py) demonstrates how`
`136`	`136`	`to create a simple MIDI device to send MIDI data to and from the USB host.`
	`137`	`+`
	`138`	`+### Limitations`
	`139`	`+`
	`140`	`+#### Buffer thread safety`
	`141`	`+`
	`142`	`+The internal Buffer class that's used by most of the USB device classes expects data`
	`143`	`+to be written to it (i.e. sent to the host) by only one thread. Bytes may be`
	`144`	`+lost from the USB transfers if more than one thread (or a thread and a callback)`
	`145`	`+try to write to the buffer simultaneously.`
	`146`	`+`
	`147`	`+If writing USB data from multiple sources, your code may need to add`
	`148`	`+synchronisation (i.e. locks).`

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`‎micropython/usb/README.md‎`

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