media: v4l2-event.rst: improve events description

Both v4l2-event.rst and v4l2-event.h have an overview of
events, but there are some inconsistencies there:

- at v4l2-event, the event's ring buffer is called kevent. Its
  name is, instead, v4l2_kevent;

- Some things are mentioned on both places (with different words),
  others are either on one of the files.

In order to cleanup this mess, put everything at v4l2-event.rst
and improve it to be a little more coherent and to have cross
references.

Acked-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>