media: docs: add documentation for frontend attach info

Add to the media book the attachment kAPI for the DVB
frontend drivers that have already some kernel-doc markup.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/dvb-drivers/frontends.rst

+****************
+Frontend drivers
+****************
+
+Frontend attach headers
+***********************
+
+.. Keep it on alphabetic order
+
+.. kernel-doc:: drivers/media/dvb-frontends/a8293.h
+.. kernel-doc:: drivers/media/dvb-frontends/af9013.h
+.. kernel-doc:: drivers/media/dvb-frontends/ascot2e.h
+.. kernel-doc:: drivers/media/dvb-frontends/cxd2820r.h
+.. kernel-doc:: drivers/media/dvb-frontends/drxk.h
+.. kernel-doc:: drivers/media/dvb-frontends/dvb-pll.h
+.. kernel-doc:: drivers/media/dvb-frontends/helene.h
+.. kernel-doc:: drivers/media/dvb-frontends/horus3a.h
+.. kernel-doc:: drivers/media/dvb-frontends/ix2505v.h
+.. kernel-doc:: drivers/media/dvb-frontends/m88ds3103.h
+.. kernel-doc:: drivers/media/dvb-frontends/mb86a20s.h
+.. kernel-doc:: drivers/media/dvb-frontends/mn88472.h
+.. kernel-doc:: drivers/media/dvb-frontends/rtl2830.h
+.. kernel-doc:: drivers/media/dvb-frontends/rtl2832.h
+.. kernel-doc:: drivers/media/dvb-frontends/rtl2832_sdr.h
+.. kernel-doc:: drivers/media/dvb-frontends/stb6000.h
+.. kernel-doc:: drivers/media/dvb-frontends/tda10071.h
+.. kernel-doc:: drivers/media/dvb-frontends/tda826x.h
+.. kernel-doc:: drivers/media/dvb-frontends/zd1301_demod.h
+.. kernel-doc:: drivers/media/dvb-frontends/zl10036.h
+

Documentation/media/dvb-drivers/index.rst

@@ -41,4 +41,5 @@ For more details see the file COPYING in the source distribution of Linux.
 	technisat
 	ttusb-dec
 	udev
+	frontends
 	contributors