[media] DocBook: better document FE_DISEQC_SEND_BURST ioctl

Use the proper format for FE_DISEQC_SEND_BURST documentation
and  improve the documentation.

Keep the enum fe_sec_mini_cmd description together with
the ioctl, as both are used together.

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