[media] DocBook: Improve the description of the properties API

<section id="FE_GET_SET_PROPERTY">

<title><constant>FE_GET_PROPERTY/FE_SET_PROPERTY</constant></title>

<para>This section describes the DVB version 5 extension of the DVB-API, also

called "S2API", as this API were added to provide support for DVB-S2. It was

designed to be able to replace the old frontend API. Yet, the DISEQC and

<section id="frontend-properties">

<title>DVB Frontend properties</title>

<para>Tuning into a Digital TV physical channel and starting decoding it

    requires to change a set of parameters, in order to control the

    tuner, the demodulator, the Linear Low-noise Amplifier (LNA) and to set the

    antena subsystem via Satellite Equipment Control (SEC), on satellital

    systems. The actual parameters are specific to each particular digital

    TV standards, and may change as the digital TV specs evolutes.</para>

<para>In the past, the strategy used were to have an union with the parameters

    needed to tune for DVB-S, DVB-C, DVB-T and ATSC delivery systems grouped

    there. The problem is that, as the second generation standards appeared,

    those structs were not big enough to contain the additional parameters.

    Also, the union didn't have any space left to be expanded without breaking

    userspace. So, the decision was to deprecate the legacy union/struct based

    approach, in favor of a properties set approach.</para>

<para>By using a properties set, it is now possible to extend and support any

    digital TV without needing to redesign the API</para>

<para>Example: with the properties based approach, in order to set the tuner

    to a DVB-C channel at 651 kHz, modulated with 256-QAM, FEC 3/4 and symbol

    rate of 5.217 Mbauds, those properties should be sent to

    <link linkend="FE_GET_PROPERTY"><constant>FE_SET_PROPERTY</constant></link> ioctl:</para>

    <itemizedlist>

	<listitem>DTV_FREQUENCY = 651000000</listitem>

	<listitem>DTV_MODULATION = QAM_256</listitem>

	<listitem>DTV_INVERSION = INVERSION_AUTO</listitem>

	<listitem>DTV_SYMBOL_RATE = 5217000</listitem>

	<listitem>DTV_INNER_FEC = FEC_3_4</listitem>

	<listitem>DTV_TUNE</listitem>

    </itemizedlist>

<para>NOTE: This section describes the DVB version 5 extension of the DVB-API,

also called "S2API", as this API were added to provide support for DVB-S2. It

was designed to be able to replace the old frontend API. Yet, the DISEQC and

the capability ioctls weren't implemented yet via the new way.</para>

<para>The typical usage for the <constant>FE_GET_PROPERTY/FE_SET_PROPERTY</constant>

API is to replace the ioctl's were the <link linkend="dvb-frontend-parameters">