[media] docs-rst: move cec kAPI documentation to the media book

The CEC kAPI documentation should also be part of the media book.
Signed-off-by: NMauro Carvalho Chehab <mchehab@s-opensource.com>

Documentation/media/media_kapi.rst

@@ -32,3 +32,4 @@ For more details see the file COPYING in the source distribution of Linux.
     kapi/dtv-core
     kapi/rc-core
     kapi/mc-core
+    kapi/cec-core