Update mesh related configuration help texts

-Update mbed-mesh-api nanostack heap-size configuration help
-Add WISUN to netsocket default-mesh-type configuration help

Author: Arto Kinnunen

features/nanostack/mbed-mesh-api/mbed_lib.json

@@ -3,7 +3,7 @@
     "requires": ["nanostack"],
     "config": {
         "heap-size": {
-            "help": "Nanostack's heap size [bytes: 0-65534]",
+            "help": "Nanostack's heap size [bytes: 0-‭4294967295‬]",
             "value": 32500
         },
         "use-malloc-for-heap": {

features/netsocket/mbed_lib.json

@@ -39,7 +39,7 @@
             "value": null
         },
         "default-mesh-type": {
-            "help": "Configuration type for MeshInterface::get_default_instance(). [LOWPAN/THREAD]",
+            "help": "Configuration type for MeshInterface::get_default_instance(). [LOWPAN/THREAD/WISUN]",
             "value": "THREAD"
         },
         "dns-response-wait-time": {