[media] v4l2-event.rst: add text from v4l2-framework.rst

[media] v4l2-event.rst: add text from v4l2-framework.rst

Move the v4l2 event-specific text from v4l2-framework.rst
to v4l2-event.rst. That helps to keep the text together with
the functions it describes, and makes easier to identify
documentation gaps.

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