[media] DocBook: add dvb_math.h to documentation

There are already some comments at dvb_math.h that are ready
for DocBook, although not properly formatted.

Convert them, fix some issues and add this file to
the device-drivers DocBook.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>
Acked-by: Jonathan Corbet <corbet@lwn.net>

Documentation/DocBook/device-drivers.tmpl

@@ -229,6 +229,7 @@ X!Isound/sound_firmware.c
 !Iinclude/media/rc-core.h
 !Idrivers/media/dvb-core/dvb_ca_en50221.h
 !Idrivers/media/dvb-core/dvb_frontend.h
+!Idrivers/media/dvb-core/dvb_math.h
 <!-- FIXME: Removed for now due to document generation inconsistency
 X!Iinclude/media/v4l2-ctrls.h
 X!Iinclude/media/v4l2-dv-timings.h
@@ -241,7 +242,6 @@ X!Edrivers/media/dvb-core/dvb_demux.c
 X!Idrivers/media/dvb-core/dvbdev.h
 X!Edrivers/media/dvb-core/dvb_net.c
 X!Idrivers/media/dvb-core/dvb_ringbuffer.h
-X!Idrivers/media/dvb-core/dvb_math.h
 -->
 
   </chapter>