media: ca docs: document CA_SET_DESCR ioctl and structs

``msg``

  Pointer to struct :c:type:`ca_descr`.

.. c:type:: ca_descr

.. code-block:: c

    struct ca_descr {

	unsigned int index;

	unsigned int parity;

	unsigned char cw[8];

    };

Description

-----------

.. note:: This ioctl is undocumented. Documentation is welcome.

CA_SET_DESCR is used for feeding descrambler CA slots with descrambling

keys (refered as control words).

Return Value

------------

	unsigned char msg[256];

};

/**

 * struct ca_descr - CA descrambler control words info

 *

 * @index: CA Descrambler slot

 * @parity: control words parity, where 0 means even and 1 means odd

 * @cw: CA Descrambler control words

 */

struct ca_descr {

	unsigned int index;

	unsigned int parity;	/* 0 == even, 1 == odd */

	unsigned int parity;

	unsigned char cw[8];

};