[media] doc-rst: Split v4l-core into one file per kAPI

Sphinx produce a 1:1 mapping between a rst file and an html file.

So, we need to split the kernel-doc tags on multiple documents.

A side effect is that we're now having a better name for each
section of the kAPI documentation.

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

Documentation/media/kapi/v4l2-event.rst

@@ -0,0 +1,4 @@
+V4L2 event kAPI
+^^^^^^^^^^^^^^^
+
+.. kernel-doc:: include/media/v4l2-event.h