media: dvbproperty.rst: improve notes about legacy frontend calls

systems. The actual parameters are specific to each particular digital

TV standards, and may change as the digital TV specs evolves.

In the past, the strategy used was to have a 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.

In the past (up to DVB API version 3), the strategy used was to have a

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, the size of such union was not big

enough to group the structs that would be required for those new

standards. Also, extending it would break userspace.

.. note::

So, the legacy union/struct based approach was deprecated, in favor

of a properties set approach.

   On Linux DVB API version 3, setting a frontend were done via

   struct :c:type:`dvb_frontend_parameters`.

   This got replaced on version 5 (also called "S2API", as this API were

   added originally_enabled to provide support for DVB-S2), because the

   old API has a very limited support to new standards and new hardware.

This section describes the new and recommended way to set the frontend,

with suppports all digital TV delivery systems.

.. note::

   1. On Linux DVB API version 3, setting a frontend was done via

      struct :c:type:`dvb_frontend_parameters`.

   2. Don't use DVB API version 3 calls on hardware with supports

      newer standards. Such API provides no suport or a very limited

      support to new standards and/or new hardware.

   3. Nowadays, most frontends support multiple delivery systems.

      Only with DVB v5 calls it is possible to switch between

      the multiple delivery systems supported by a frontend.

   4. DVB API version 5 is also called *S2API*, as the first

      new standard added to it was DVB-S2.

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