examples/provisioning: add comment about linker errors

If the user copies the BLE example to their own project without understanding the need to enable the BT stack or BTDM BLE settings in the SDK then their build will probably fail at this line due to linker errors.

Closes https://github.com/espressif/esp-idf/pull/6652
Closes IDFGH-4854

examples/provisioning/wifi_prov_mgr/main/app_main.c

@@ -271,6 +271,10 @@ void app_main(void)
             0xb4, 0xdf, 0x5a, 0x1c, 0x3f, 0x6b, 0xf4, 0xbf,
             0xea, 0x4a, 0x82, 0x03, 0x04, 0x90, 0x1a, 0x02,
         };
+
+        /* If your build fails with linker errors at this point, then you may have
+         * forgotten to enable the BT stack or BTDM BLE settings in the SDK (e.g. see
+         * the sdkconfig.defaults in the example project) */
         wifi_prov_scheme_ble_set_service_uuid(custom_service_uuid);
 #endif /* CONFIG_EXAMPLE_PROV_TRANSPORT_BLE */