media: dmx-get-pes-pids.rst: document the ioctl

    File descriptor returned by :c:func:`open() <dvb-dmx-open>`.

``pids``

    Undocumented.

    Array used to store 5 Program IDs.

Description

-----------

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

This ioctl allows to query a DVB device to return the first PID used

by audio, video, textext, subtitle and PCR programs on a given service.

They're stored as:

=======================	========	=======================================

PID  element		position	content

=======================	========	=======================================

pids[DMX_PES_AUDIO]	0		first audio PID

pids[DMX_PES_VIDEO]	1		first video PID

pids[DMX_PES_TELETEXT]	2		first teletext PID

pids[DMX_PES_SUBTITLE]	3		first subtitle PID

pids[DMX_PES_PCR]	4		first Program Clock Reference PID

=======================	========	=======================================

.. note::

	A value equal to 0xffff means that the PID was not filled by the

	Kernel.

Return Value