Improve WisunInterface set_network_size docs

Clarify set_network_size usage and possible parameter values.

Author: Arto Kinnunen

features/nanostack/mbed-mesh-api/mbed-mesh-api/WisunInterface.h

@@ -185,15 +185,22 @@ class WisunInterface : public MeshInterfaceNanostack {
     /**
      * \brief Set Wi-SUN network size.
      *
-     * Function stores new parameters to mbed-mesh-api and uses them when connect() is called next time.
-     * If device is already connected to the Wi-SUN network then device will restart network discovery after
+     * Function stores network size parameter to the mbed-mesh-api and uses it when connect() is called for the next
+     * time. If device is already connected to the Wi-SUN network then device will restart network discovery after
      * changing the network size.
      *
-     * Default value: medium
-     * Small network size: less than hundred devices
-     * Medium network size: hundreds of devices
-     * Large network size: thousands of devices
-     * Certificate: used on testing
+     * It is recommended to set the correct network size because the Wi-SUN network configuration parameters are
+     * adjusted based on the selected network size. A network configured for a small amount of devices may not work
+     * optimally for large amount of devices.
+     *
+     * The network size is measured as hundreds of devices. For example:
+     * - set network size to 1 to use Wi-SUN network with less than 100 devices (small).
+     * - set network size to 8 to use Wi-SUN network with 100-800 devices (medium).
+     * - set network size to 15 to use Wi-SUN network with 800-1500 devices (large).
+     * - set network size to 25 to use Wi-SUN network with more than 2500 devices (xlarge).
+     * - set network size to 0 to run certification tests.
+     *
+     * By default Wi-SUN stack is configured to use medium sized network.
      *
      * When network size is changed, it will override all or some of the following configuration values:
      * - Timing settings set by set_timing_parameters() of the Wi-SUN interface.
@@ -202,7 +209,7 @@ class WisunInterface : public MeshInterfaceNanostack {
      * When network size is changed, and if timing or RPL values should be other than defaults set by stack for the network size,
      * they need to set again using above function calls.
      *
-     * \param network_size Network size in hundreds of devices (e.g. 1200 devices is 12), 0x00 for network size certificate.
+     * \param network_size Network size in hundreds of devices (e.g. 1200 devices is 12), 0 for certificate testing.
      * \return MESH_ERROR_NONE on success.
      * \return MESH_ERROR_UNKNOWN in case of failure.
      * */