Add note about MEMP_NUM_SYS_TIMEOUT in four apps

doc/mqtt_client.txt

@@ -8,6 +8,8 @@ http://docs.oasis-open.org/mqtt/mqtt/v3.1.1/os/mqtt-v3.1.1-os.html
 -----------------------------------------------------------------
 1. Initial steps, reserve memory and make connection to server:
 
+You need to increase MEMP_NUM_SYS_TIMEOUT by one if you use MQTT!
+
 1.1: Provide storage
 
 Static allocation:

src/apps/mdns/mdns.c

@@ -8,6 +8,8 @@
  * RFC 6762 - Multicast DNS\n
  * RFC 6763 - DNS-Based Service Discovery\n
  *
+ * You need to increase MEMP_NUM_SYS_TIMEOUT by one if you use MDNS!
+ *
  * @verbinclude mdns.txt
  *
  * Things left to implement:

src/apps/sntp/sntp.c

@@ -42,6 +42,8 @@
  * This is simple "SNTP" client for the lwIP raw API.
  * It is a minimal implementation of SNTPv4 as specified in RFC 4330.
  *
+ * You need to increase MEMP_NUM_SYS_TIMEOUT by one if you use SNTP!
+ *
  * For a list of some public NTP servers, see this link:
  * http://support.ntp.org/bin/view/Servers/NTPPoolServers
  *

src/apps/tftp/tftp.c

@@ -45,6 +45,7 @@
  * @ingroup apps
  *
  * This is simple TFTP client/server for the lwIP raw API.
+ * You need to increase MEMP_NUM_SYS_TIMEOUT by one if you use TFTP!
  */
 
 #include "lwip/apps/tftp_client.h"