[spi] added comment about buffer length requirements for stm32

sw/airborne/mcu_periph/spi.h

@@ -135,6 +135,9 @@ typedef void (*SPICallback)( struct spi_transaction *trans );
  *   of the two specifies the toal number of exchanged bytes,
  * - if input_length is larger than output length,
  *   0 is sent for the remaining bytes
+ * WARNING: For STM32 only, the output_buf size MUST be greater than or equal
+ * to the input_buf size. This is only required in the event any transaction
+ * has (0 < output_length < input_length).
  */
 struct spi_transaction {
   volatile uint8_t* input_buf;