Document implicit assumption in non-blocking spi trait

src/spi.rs

@@ -8,7 +8,13 @@ use nb;
 ///
 /// - It's the task of the user of this interface to manage the slave select lines
 ///
-/// - Due to how full duplex SPI works each `read` call must be preceded by a `send` call.
+/// - Due to how full duplex SPI works each `try_read` call must be preceded by a `try_send` call.
+///
+/// - `try_read` calls only return the data received with the last `try_send` call.
+/// Previously received data is discarded
+///
+/// - Data is only guaranteed to be clocked out when the `try_read` call succeeds.
+/// The slave select line shouldn't be released before that.
 ///
 /// - Some SPIs can work with 8-bit *and* 16-bit words. You can overload this trait with different
 /// `Word` types to allow operation in both modes.