Fix several comments for clang -Wdocumentation

apps/smtp/smtp.c

@@ -574,7 +574,8 @@ leave:
  * @param to target email address (must be NULL-terminated)
  * @param subject email subject (must be NULL-terminated)
  * @param body email body (must be NULL-terminated)
- * @param 
+ * @param callback_fn callback function
+ * @param callback_arg user argument to callback_fn
  * @returns - ERR_OK if structures were allocated and no error occured starting the connection
  *            (this does not mean the email has been successfully sent!)
  *          - another err_t on error.

ports/unix/port/include/netif/fifo.h

@@ -23,7 +23,7 @@ typedef struct fifo_t {
 /**
 *   Get a character from fifo
 *   Blocking call.
-*	@param 	pointer to fifo data structure
+*	@param 	fifo pointer to fifo data structure
 *	@return	character read from fifo
 */
 u8_t fifoGet(fifo_t * fifo);
@@ -31,7 +31,7 @@ u8_t fifoGet(fifo_t * fifo);
 /**
 *   Get a character from fifo
 *   Non blocking call.
-*	@param 	pointer to fifo data structure
+*	@param 	fifo pointer to fifo data structure
 *	@return	character read from fifo, or < zero if non was available
 */
 s16_t fifoGetNonBlock(fifo_t * fifo);

ports/unix/port/netif/sio.c

@@ -98,8 +98,9 @@ static void signal_handler_IO_1( int status )
 
 /**
 * Initiation of serial device 
-* @param device : string with the device name and path, eg. "/dev/ttyS0"
-* @param netif  : netinterface struct, contains interface instance data
+* @param device string with the device name and path, eg. "/dev/ttyS0"
+* @param devnum device number
+* @param siostat status
 * @return file handle to serial dev.
 */
 static int sio_init( char * device, int devnum, sio_status_t * siostat )