[media] DocBook: Add an example for using FE_SET_PROPERTY

In order to make it clearer about how to use the DVBv5 calls,
add an example of its usage. That should make it clearer about
what's actually required for the DVBv5 calls to work.

While here, also mentions the libdvbv5 library.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>