Add/update docs

docs/api-reference/apps/mqtt_client.rst

@@ -3,5 +3,11 @@
 MQTT Client
 ===========
 
+MQTT client v3.1.1 implementation, based on callback (non-netconn) connection API.
+
+.. literalinclude:: ../../../snippets/mqtt_client.c
+    :language: c
+    :caption: MQTT application example code
+
 .. doxygengroup:: ESP_APP_MQTT_CLIENT
 .. doxygengroup:: ESP_APP_MQTT_CLIENT_EVT

docs/api-reference/apps/mqtt_client_api.rst

@@ -3,4 +3,10 @@
 MQTT Client API
 ===============
 
+*MQTT Client API* provides sequential API built on top of :ref:`api_app_mqtt_client`.
+
+.. literalinclude:: ../../../snippets/mqtt_client_api.c
+    :language: c
+    :caption: MQTT API application example code
+
 .. doxygengroup:: ESP_APP_MQTT_CLIENT_API

docs/user-manual/porting-guide.rst

@@ -16,9 +16,47 @@ Porting consists of:
 Implement low-level part
 ^^^^^^^^^^^^^^^^^^^^^^^^
 
+To successfully implement all parts of *low-level* driver, application must take care of:
+
+* Implementing :cpp:func:`esp_ll_init` and :cpp:func:`esp_ll_deinit` callback functions
+* Implement and assign *send data* and optional *hardware reset* functions callbacks
+* Assign memory for allocation manager when using default allocator
+* Process received data from *ESP* device and send them to input module for further processing
+
+.. tip::
+    Port examples are available for STM32 and WIN32 architectures.
+    Both actual working and up-to-date implementations are available within the library.
+
+.. note::
+    Check :ref:`api_esp_input` for more information about direct & indirect input processing.
+
 Implement system functions
 ^^^^^^^^^^^^^^^^^^^^^^^^^^
 
+System functions are bridge between operating system calls and *ESP* middleware.
+*ESP* library relies on stable operating system features and its implementation and
+does not require any special features which do not normally come with operating systems.
+
+Operating system must support:
+
+* Thread management
+* Mutex management
+* Binary semaphores
+* Message queues
+
+.. warning::
+    If any of the features are not available within targeted operating system,
+    customer needs to resolve it with care. As an example, message queue is not available
+    in WIN32 OS API therefore custom message queue has been implemented using binary semaphores
+
+Application needs to implement all system call functions, starting with ``esp_sys_``.
+It must also prepare header file for standard types in order to support OS types within *ESP* middleware.
+
+An example code is provided latter section of this page for WIN32 and STM32.
+
+.. note::
+    Check :ref:`api_esp_sys` for function prototypes.
+
 Example: Low-level driver for WIN32
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^