fpi-{usb,spi}: Adjust docs to mention required env variables for logging

libfprint/fpi-spi-transfer.c

@@ -42,7 +42,7 @@ static gsize block_size = 0;
  * Drivers should always use this API rather than calling read/write/ioctl on
  * the spidev device.
  *
- * Setting G_MESSAGES_DEBUG and FP_DEBUG_TRANSFER will result in the message
+ * Setting %G_MESSAGES_DEBUG and %FP_DEBUG_TRANSFER will result in the message
  * content to be dumped.
  */
 

libfprint/fpi-usb-transfer.c

@@ -31,6 +31,9 @@
  *
  * Drivers should use this API only rather than accessing the GUsbDevice
  * directly in most cases.
+ *
+ * Setting %G_MESSAGES_DEBUG and %FP_DEBUG_TRANSFER will result in the message
+ * content to be dumped.
  */