docs(esp_modem): updated documents to show missed topics

docs/esp_modem/en/internal_design.rst

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+Internal design
+===============
+
+Design decisions
+----------------
+
+-  Use C++ with additional C API
+
+-  Use exceptions
+
+   -  Use macro wrapper over ``try-catch`` blocks when exceptions off
+      (use ``abort()`` if ``THROW()``)
+
+-  Initializes and allocates in the constructor (might throw)
+
+   -  easier code with exceptions ON, with exceptions OFF alloc/init
+      failures are not treated as runtime error (program aborts)
+   -  break down long initialization in constructor into more private
+      methods
+
+-  Implements different devices using inheritance from
+   ``GenericModule``, which is the most general implementation of a
+   common modem
+
+   -  Internally uses templates with device specialization (modeled as
+      ``DCE<SpecificModule>``) which could be used as well for some
+      special cases, such as implantation of a minimal device
+      (ModuleIf), add new AT commands (oOnly in compile time), or using
+      the Module with DTE only (no DCE, no Netif) for sending AT
+      commands without network
+
+DCE collaboration model
+-----------------------
+
+The diagram describes how the DCE class collaborates with DTE, PPP and
+the device abstraction
+
+.. figure:: DCE_DTE_collaboration.png
+   :alt: DCE_architecture
+
+   DCE_architecture
+
+Terminal inheritance
+--------------------
+
+Terminal is a class which can read or write data, and can handle
+callbacks when data are available. UART specialization is provided
+implementing these method using the uart driver.
+
+CMUX terminal
+-------------
+
+The below diagram depicts the idea of using CMUX terminal mode using the
+CMuxInstance class which is a terminal (it implements the basic
+read/write methods) interfacing arbitrary number of virtual terminals,
+but at the same time it is also composed of CMux class, which consumes
+the original terminal and uses its read/write methods to multiplex the
+terminal.
+
+.. figure:: CMux_collaboration.png
+   :alt: CMUX Terminal
+
+   CMUX Terminal