Warn against and check for non-DMA-capable pointers being passed to SPI when DMA is used

components/driver/include/driver/spi_slave.h

@@ -69,6 +69,10 @@ struct spi_slave_transaction_t {
  * @param dma_chan Either 1 or 2. A SPI bus used by this driver must have a DMA channel associated with
  *                 it. The SPI hardware has two DMA channels to share. This parameter indicates which
  *                 one to use.
+ *
+ * @warning If a DMA channel is selected, any transmit and receive buffer used should be allocated in 
+ *          DMA-capable memory.
+ *
  * @return 
  *         - ESP_ERR_INVALID_ARG   if configuration is invalid
  *         - ESP_ERR_INVALID_STATE if host already is in use