Updates to documentation (#280)

Documentation updates (including moving of header files

lib_xua/doc/rst/opt_audio_class.rst

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+
+USB Audio Class Version
+~~~~~~~~~~~~~~~~~~~~~~~
+
+The codebase supports USB Audio Class versions 1.0 and 2.0.
+
+USB Audio Class 2.0 offers many improvements over USB Audio Class 1.0, most notable is the complete
+support for high-speed operation.  This means that Audio Class devices are no longer limited to 
+full-speed operation allowing greater channel counts, sample frequencies and sample bit-depths. 
+Additional improvements, amongst others, include: 
+
+- Added support for multiple clock domains, clock description and clock control
+
+- Extensive support for interrupts to inform the host about dynamic changes that occur to different entities such as Clocks etc
+
+Driver Support
+..............
+
+Audio Class 1.0 
++++++++++++++++
+
+Audio Class 1.0 is fully supported in Apple OSX.  Audio Class 1.0 is fully supported in all modern Microsoft Windows operating systems (i.e. Windows XP and later). 
+
+Audio Class 2.0
++++++++++++++++
+
+Audio Class 2.0 is fully supported in Apple OSX since version 10.6.4.  Starting with Windows 10, release 1703, a USB Audio 2.0 driver is shipped with Windows. 
+
+Third party Windows drivers are also available, however, documentation of these is beyond the scope of this document, please contact XMOS for further details.
+
+Audio Class 1.0 Mode and Fall-back
+..................................
+
+The default for XMOS USB Audio applications is to run as a high-speed Audio Class 2.0
+device. However, some products may prefer to run in Audio Class 1.0 mode, this is normally to 
+allow "driver-less" operation with older versions of Windows operating systems. 
+
+.. note::
+
+    To ensure specification compliance, Audio Class 1.0 mode *always* operates at full-speed USB. 
+
+The device will operate in full-speed Audio Class 1.0 mode if one of the following is true:
+
+-  The code is compiled for USB Audio Class 1.0 only.
+
+-  The code is compiled for USB Audio Class 2.0 and it is connected
+   to the host over a full speed link (and the Audio Class fall back is 
+   enabled).
+
+The options to control this behavior are detailed in :ref:`sec_api_defines`. 
+
+When running in Audio Class 1.0 mode the following restrictions are applied:
+
+- MIDI is disabled.
+
+- DFU is disabled (Since Windows operating systems would prompt for a DFU driver to be installed)
+
+Due to bandwidth limitations of full-speed USB the following sample-frequency restrictions are also applied:
+
+-  Sample rate is limited to a maximum of 48kHz if both input and output are enabled.
+
+-  Sample rate is limited to a maximum of 96kHz if only input *or* output is enabled.
+
+  
+Related Defines
+................
+
+:ref:`opt_audio_class_defines` descibes the defines that effect audio class selection.
+
+.. _opt_audio_class_defines:
+
+.. list-table:: Audio Class defines
+   :header-rows: 1
+   :widths: 20 80 20
+
+   * - Define
+     - Description
+     - Default
+   * - ``AUDIO_CLASS``
+     - Audio Class version (1 or 2)
+     - N/A (*must* be defined) 
+   * - ``AUDIO_CLASS_FALLBACK``
+     - Enable audio class fallback functionalty 
+     - ``0`` (disabled) 
+
+.. note:: 
+
+    Enabling USB Audio Class fallback functionality may have USB Compliance implications 
+