media: dtv-core.rst: add chapters and introductory tests for common parts

Better document the DVB common parts by adding two sections
and an introductory text for each.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/kapi/dtv-core.rst

@@ -29,8 +29,20 @@ I2C bus.
 Digital TV Common functions
 ---------------------------
 
+Math functions
+~~~~~~~~~~~~~~
+
+Provide some commonly-used math functions, usually required in order to
+estimate signal strength and signal to noise measurements in dB.
+
 .. kernel-doc:: drivers/media/dvb-core/dvb_math.h
 
+
+DVB devices
+~~~~~~~~~~~
+
+Those functions are responsible for handling the DVB device nodes.
+
 .. kernel-doc:: drivers/media/dvb-core/dvbdev.h
 
 Digital TV Ring buffer