Comment clarity

[joegittingherdone]

I see lines:

nmbs->platform.read(nmbs->msg.buf, sizeof(nmbs->msg.buf), 0, nmbs->platform.arg);

Should the user's read and write callbacks empty the nmbs->msg.buf and the hardware's UART buffers when the timeout parameter is zero? The comment in nanomodbus.h doesn't say to do this:

Both methods should block until either:
- count bytes of data are read/written
- the byte timeout, with byte_timeout_ms >= 0, expires