spi: incorporate change from GH issue feedback

Signed-off-by: deadprogram <ron@hybridgroup.com>

spi.go

@@ -2,6 +2,12 @@ package drivers
 
 // SPI represents a SPI bus. It is implemented by the machine.SPI type.
 type SPI interface {
+	// Tx transmits the given buffer w and receives at the same time the buffer r.
+	// The two buffers must be the same length. The only exception is when w or r are nil,
+	// in which case Tx only transmits (without receiving) or only receives (while sending 0 bytes).
 	Tx(w, r []byte) error
+
+	// Transfer writes a single byte out on the SPI bus and receives a byte at the same time.
+	// If you want to transfer multiple bytes, it is more efficient to use Tx instead.
 	Transfer(b byte) (byte, error)
 }