Documentation/components/drivers/character/wireless: IOCTL interface initial documentation.

Adds initial documentation to the IOCTL commands of wireless character devices.

Signed-off-by: Kevin Witteveen (MartiniMarter) <kevinwit1999@gmail.com>

Documentation/components/drivers/character/wireless/index.rst

@@ -6,3 +6,48 @@ Wireless character drivers
   :maxdepth: 1
 
   lpwan/index.rst
+
+IOCTL Interface
+===============
+
+Before this interface, the IOCTL API for character-driven RF devices lacked
+a common interface across different modulation technologies,
+such as LoRa, FSK, and OOK. The result was, driver-specific IOCTL commands
+were created even when they could be shared across multiple radios.
+This fragmentation made application portability more difficult to maintain.
+
+To solve this issue, groups of commands have been created that are "selected"
+by using ``WLIOC_SETMODU``.
+See image below for a representation.
+
+.. figure:: wireless_ioctl_white.drawio.png
+
+.. note:: This image does not show all features. This is purely a
+          representation of command relations under WLIOC_SETMODU.
+
+read()
+------
+
+Reading a radio will take a ``wlioc_rx_hdr_s``, where information about the
+payload will be read and written to.
+
+- ``FAR uint8_t *payload_buffer`` Pointer to **user buffer**. This is where
+  the payload will be written to.
+- ``size_t payload_length`` **Initially**: User must set this to the size
+  of ``payload_buffer``. **After reading**: This will become the amount of
+  bytes written to the ``payload_buffer``.
+- ``uint8_t error`` When greater than 0, there are errors detected in the
+  payload. The payload can still be returned, which allows the user to repair
+  it if desired.
+- ``int32_t rssi_dbm`` The Received Signal Strength Indicator in
+  1/100 decibel-milliwatts of the received signal.
+  When unsupported, this returns ``INT32_MIN``.
+- ``int32_t snr_db`` The signal to noise ratio in 1/100 decibels of the
+  received signal. When unsupported, this returns ``INT32_MIN``.
+
+write()
+-------
+
+Writing to a radio will attempt to send the given bytes. The radio must be
+configured before doing so.
+Unlike ``read()``, this will simply take uint8_t bytes as payload.